Unusual Engine Behavior After Hot Start

[lr172]

What's causing the MAP to pulse?

Vaccuum is good indicator of the proper combustion process at idle. Both timing and mixture will impact the level of vacuum produced by the engine at idle, as will valve issues, though usually to a larger degree. For example, I set my idle mixture by looking for the highest vacuum (lowest MAP). As I lean the mixture, MAP can go down by as much as 2"; Have seen more with really rich mixtures. My speculation is that #2 is going agressively lean or rich; Cause unknown. This substantially drops the combustion performance of #2 and this increases the MAP, which is additive from all cylinders. You can see the MAP change in perfect harmony with the EGT in his charts, which is what points me to a combustion efficiency issue. SOMETHING is causing combustion efficiency to drop.

Go research from the 70's. Very common practice to use vaccuum gauges to troubleshoot engines. You will see several pages of various scenarios / behavior of the gauge and what problem it translates to. Vaccuum level is a fabulaous tool and provides many clues, especially idle issues. It is also great for identifying valve issues, even at higher power settings.

 

[bjdecker]

Vaccuum is good indicator of the proper combustion process at idle. Both timing and mixture will impact the level of vacuum produced by the engine at idle. For example, I set my idle mixture by looking for the highest vacuum (lowest MAP). As I lean the mixture, MAP can go down by as much as 2"; Have seen more with really rich mixtures. My speculation is that #2 is going agressively lean or rich. This substantially drops the combustion performance of #2 and this drops the MAP, which is additive from all cylinders.

Go research from the 70's. Very common practice to use vaccuum gauges to troubleshoot engines. You will see several pages of various scenarios / behavior of the gauge and what problem it translates to. Vaccuum level is a fabulaous tool and provides many clues, especially idle issues.

The MAP isn't "dropping", it's increasing toward ambient ... I asked Michael to post the link so Savvy so we can get a better look that these parameters.

 

[lr172]

The MAP isn't "dropping", it's increasing toward ambient ... I asked Michael to post the link so Savvy so we can get a better look that these parameters.

I had a typo and said "drops" in error. I fixed that. Was thinking vac.

Yes, rising MAP is lowering vaccuum. the closer you get to optimal combustion efficiency, the higher the vaccuum will be and the lower the MAP will be, a rising MAP at idle is indicative of LESS combustion efficiency.

 

[blaplante]

Here's a graph inflight where I flew approach speed with the throttle pulled back to an idle. and then again on approach, I pulled the throttle back to an idle. the #2 EGT act suspicious.

View attachment 97895

Seems to support my thought of debris in the spider's slot. As Larry said, unsure how this affects 2200 rpm, but once you find this issue it might explain things. Or not.

Also, for fun, as a separate test (so NOT at same time that the spider has been messed with) ... try swapping the #2 injector with another one.

At this point I'd be sending the spider off to Don at Air Flow Performance for testing. Send him this graph.

Lastly we can't discount #2 valves ... as have already given issues despite the low engine hours.

 

[Mikeyb]

You may recall this thread. It’s interesting that this engine is exhibiting the same periodic fluctuation of #2 at idle.

Slightly fluctuating EGT and lagging CHT on startup on cylinder 2

Hello, I have a very subtle problem that I noticed the other day when I was in normalize mode at startup. It looks like EGT on #2 fluctuates a little and CHT lags a bit behind the rest of the cylinders. Can't feel much seat of the pants but if my tip up prop is resting slightly open on its...

vansairforce.net

 

[mfleming]

Seems to support my thought of debris in the spider's slot. As Larry said, unsure how this affects 2200 rpm, but once you find this issue it might explain things. Or not.

Also, for fun, as a separate test (so NOT at same time that the spider has been messed with) ... try swapping the #2 injector with another one.

At this point I'd be sending the spider off to Don at Air Flow Performance for testing. Send him this graph.

Lastly we can't discount #2 valves ... as have already given issues despite the low engine hours.

I just got off the phone with Avstar. They recommend I send the spider in for a check. One of the symptoms that made Avstar recommend sending it in, is the engine recently started dieseling when shutting it down when it's hot.

They do not recommend disassembling the spider in the field.

 

[[email protected]]

Do an extended mag check to isolate the plugs / ignition wiring / coil on #2

 

[mfleming]

Do an extended mag check to isolate the plugs / ignition wiring / coil on #2

Good advice. The left mag is a Slick impulse and the right is a P-Mag. When doing a mag check and grounding the Slick, that causes a 150-160 RPM drop. I have never been able to get the P-Mag to drop less. When grounding the P-Mag, the RPM drops about 90 to 100. The jumper is in and it's timed about 2 teeth after TDC.

 

[Hitt7A]

Mine does similarly after a hot start, it's from boiling fuel in the lines and goes away under full power, or while taxing I lean the mixture way down. Running the boost pump does not solve the boiling fuel issue for me, but I did just recently find after over 500 hrs in service that aggressively leaning the mixture things smooth out and it idles just fine.

 

[mfleming]

Mine does similarly after a hot start, it's from boiling fuel in the lines and goes away under full power, or while taxing I lean the mixture way down. Running the boost pump does not solve the boiling fuel issue for me, but I did just recently find after over 500 hrs in service that aggressively leaning the mixture things smooth out and it idles just fine.

I've heard of this happening. My concern is that this is a new issue and that something changed. Talking to Savvy and Avstar, they both think somethings going on.

 

[georgemohr]

I've heard of this happening. My concern is that this is a new issue and that something changed. Talking to Savvy and Avstar, they both think somethings going on.

Agree, this looks too rhythmic for it to be boiling fuel. That is a problem we get also, but its more "chaotic" and also affects all cylinders.

 

[mfleming]

Here's a shared link to the live engine data where you can change parameters (I think)

• Ground run with engine up to temp - Graph
• Inflight - Graph

 

[lr172]

I just got off the phone with Avstar. They recommend I send the spider in for a check. One of the symptoms that made Avstar recommend sending it in, is the engine recently started dieseling when shutting it down when it's hot.

They do not recommend disassembling the spider in the field.

If the piston is getting hung up and not fully going down, it will result in excessively rich idle mixture, which I feel may be your case. However not sure that this would cause dieseling. That would seem to be more related to an issue inside the servo. Suggest getting input from them on this as well. If you just send in the spider and they don't find an issue, then they will ask you to send in the servo, wasting time. May want to call Don at AFP for a second opinion and quote. He is absolutely the most knowledgeable guy around on these parts and likely better rates.

 

[georgemohr]

The Airflow manual does mention that the spider's role is to cleanly cut off the fuel at full lean mixture. There is always residual pressure downstream of the servo that can't be completely eliminated, so the spring valve at the base of the spider provides that minimum pressure threshold to allow idle. But this really doesn't seem to solve for an issue in only one cylinder, right?

All this MAP fluctuation really looks like an intake leak of some kind to my eye.

 

[jabarr]

1) Intake leak- don’t just look at them. Change the seals. Easy enough to do.
2) Valve sticking. Do the wobble test on both valves. SB 388C. Much easier if you get a Valve Wizard. https://valvewizard.com/
3) Lifter issue- not high on my list of possibilities.

 

[Full Throttle]

I just got off the phone with Avstar. They recommend I send the spider in for a check. One of the symptoms that made Avstar recommend sending it in, is the engine recently started dieseling when shutting it down when it's hot.

They do not recommend disassembling the spider in the field.

Of course not. They can't charge you if ya fix it yourself.

 

[georgemohr]

You'll also notice that EGT #1 also moves with the same frequency, but less amplitude... This suggests something that is partially shared between them, and suggests away from something very specific (like the valves for example).

 

[sansoneservices]

Fuel is vaporizing from heat. Go to 70% with boost pump on, briefly richen mixture (don't foul your plugs) this will bring cool fuel to the engine.
Edit: reading further into the thread it appears there's more than just vaporized fuel in the lines. Good responses

 

[bjdecker]

I had a typo and said "drops" in error. I fixed that. Was thinking vac.

Yes, rising MAP is lowering vaccuum. the closer you get to optimal combustion efficiency, the higher the vaccuum will be and the lower the MAP will be, a rising MAP at idle is indicative of LESS combustion efficiency.

"The faster it spins the harder it sucks." (translation -- all things being equal, an increase in combustion efficiency which causes the ICPP to increase, causes the RPM to increase, which in turn causes a reduction in Induction Pressure - behind the Throttle Venturi/Intake plenum.)

But it's not 2" ...

Just for grins/giggles - I tried an experiment on my IO-360-A1B6:
Idle speed at 950rpm with the mixture full rich. MAP was 10.9" (field elevation is 780 MSL and Baro was 29.91"), fuel flow at 1.2gph.

Slowly turning the mixture out (Leaning), I observed the MAP drop to 10.1" and the RPM came up to 1010 and then began to die.

You'll also notice that EGT #1 also moves with the same frequency, but less amplitude... This suggests something that is partially shared between them, and suggests away from something very specific (like the valves for example).

Yup - #1 and #2 share the same cam lobe, as do #3 and #4.

 

[georgemohr]

Yup - #1 and #2 share the same cam lobe, as do #3 and #4.

Well thats interesting and I didnt know that. I guess a possibility is a failing cam lobe... but wouldn't that then show the same amplitude on both cylinders? And wouldn't it be happening regardless of heat?

 

[blaplante]

Mine does similarly after a hot start, it's from boiling fuel in the lines and goes away under full power, or while taxing I lean the mixture way down. Running the boost pump does not solve the boiling fuel issue for me, but I did just recently find after over 500 hrs in service that aggressively leaning the mixture things smooth out and it idles just fine.

But, the video and data logs only show changes in cylinder #2. Having an IO myself, I'm very familiar with what you say, but I've never seen it affect only 1 cylinder.

 

[Avanza]

I would replace the sparkplugs on cyl #2 or move them to another cylylinder just to see if there is a change.

Good luck

 

[bjdecker]

But, the video and data logs only show changes in cylinder #2. Having an IO myself, I'm very familiar with what you say, but I've never seen it affect only 1 cylinder.

There's a larger signature on #1 EGT to be sure, however #2 also shows a dip. Click on "appearance" and hide #3 and #4

 

[bjdecker]

Well thats interesting and I didnt know that. I guess a possibility is a failing cam lobe... but wouldn't that then show the same amplitude on both cylinders? And wouldn't it be happening regardless of heat?

Correct -- I don't think it's the CAM lobe. However, the Roller Tappet -- LRT23381 -- could be sick. I just can't explain the period between the drop outs -- 6 times a minute? The valves are opening at ~9.41/sec at 1130RPM, soo....I dunno...

 

[RV-9erA]

Easy to understand and very interesting theory of Lycoming mechanical fuel injection. An interesting aspect--idle and cut-off fuel flows are handled completely different than the higher power settings in both the injector servo and flow divider (spider). And in the fuel flow divider, scoring within the fuel valve assembly can actually make it appear like a single nozzle issue. Videos by the Synergy Flight Training Campus. I think the flow divider, induction leak or spark plug are the most likely cause. The engine only has 250 hours on it, and it's a Lycoming IO-360, which are well known for long reliable service life!

 

[lr172]

"The faster it spins the harder it sucks." (translation -- all things being equal, an increase in combustion efficiency which causes the ICPP to increase, causes the RPM to increase, which in turn causes a reduction in Induction Pressure - behind the Throttle Venturi/Intake plenum.)

But it's not 2" ...

Just for grins/giggles - I tried an experiment on my IO-360-A1B6:
Idle speed at 950rpm with the mixture full rich. MAP was 10.9" (field elevation is 780 MSL and Baro was 29.91"), fuel flow at 1.2gph.

Slowly turning the mixture out (Leaning), I observed the MAP drop to 10.1" and the RPM came up to 1010 and then began to die.

Yup - #1 and #2 share the same cam lobe, as do #3 and #4.

If your idle mixture is well rich, where new boxes usually are, you will get a 1- 2” drop (how much depends upon how rich it was to start) in map as you lean. However, you need to keep bringing the throttle back to the same rpm as where you started to get that drop. Otherwise you are just getting an rpm rise which naturally drops the map on its own, as you explained above. To optimize the mixture, you are looking for the lowest MAP at a constant RPM.

If you start with a proper mixture and just lean it, no you won’t get a 2” drop. If your mixture was already close to optimal, you won't get any drop or very little.

 

[mfleming]

On the advice of Avstar, I swapped the injector restrictors to see if the EGT that was dropping out (#2) would change places...it didn't. The surging is getting worse and I can consistently get it to surge. After flying, the engine idles perfectly but shut it down and restart 5 min later and it runs really bad. Today I had it surging and tried the boost pump...that made it worse. I also ran up the RPM to 2400, it didn't clear the problem and I think it missed a couple times. It then dieseled really badly on shutdown. Avstar said send the fuel distributer in and I think I'm going to pull it off tomorrow and do just that.

 

[lr172]

. Today I had it surging and tried the boost pump...that made it worse.

This points more to an issue in the servo. Again suggest you get that looked at as well. Will state again that a second opinion from don at AFP would be a good idea.

 

[mfleming]

This points more to an issue in the servo. Again suggest you get that looked at as well. Will state again that a second opinion from don at AFP would be a good idea.

I talked to AFP but didn't get Don. They suspected the fuel divider too.

 

[AZSteve053]

On the advice of Avstar, I swapped the injector restrictors to see if the EGT that was dropping out (#2) would change places...it didn't. The surging is getting worse and I can consistently get it to surge. After flying, the engine idles perfectly but shut it down and restart 5 min later and it runs really bad. Today I had it surging and tried the boost pump...that made it worse. I also ran up the RPM to 2400, it didn't clear the problem and I think it missed a couple times. It then dieseled really badly on shutdown. Avstar said send the fuel distributer in and I think I'm going to pull it off tomorrow and do just that.

Michael,
If you send the fuel divider back to AvStar... and it sounds like you certainly should - I would ALSO send the Servo so that they can bench test it alone as well as the servo & Divider together. If no problem with Servo then you'll get it back functionally tested / verified. AvStar can turn things really quickly when they want, a little prompting always helps. You might wonder WHY I suggest this.... well all the fuel getting to the divider goes thru the servo and any FOD that got into Divider came from somewhere - possibly a piece from inside Servo (bit of a diaphragm or o-ring). While your waiting to get them back - suggest pulling injectors for a Hoppe's bath so you know everything is clean. Flush and back flush all the SS divider to injector lines too. You'll sleep better afterwords.

 

[lr172]

I talked to AFP but didn't get Don. They suspected the fuel divider too.

Sounds good. Those guys would know. Did you tell them about the fact that the boost pump makes the condition worse? That is a point they need to know, as all your other symptoms point to the divider.

 

[Mikeyb]

You may recall this thread. It’s interesting that this engine is exhibiting the same periodic fluctuation of #2 at idle.

Slightly fluctuating EGT and lagging CHT on startup on cylinder 2

Hello, I have a very subtle problem that I noticed the other day when I was in normalize mode at startup. It looks like EGT on #2 fluctuates a little and CHT lags a bit behind the rest of the cylinders. Can't feel much seat of the pants but if my tip up prop is resting slightly open on its...

vansairforce.net

HIt’s interesting that both of these engines idling are indicating fuel flows of 2 to 3 GPH. mine would indicate 1.3 GPH in the same condition. Brian’s below as well.

It’s interesting that both these fluctuating idle speed engines are indicating fuel flows of 2-3 gph. Mine indicates 1.3 or so Brian below near the same.

 

[mfleming]

I talked to Avstar this morning and although they think the flow divider is all they need, we decided to send in the flow divider as well as the servo. I'm headed over to the hangar to pull them off now. They said they can do a pretty fast turn around so I shouldn't be down for too long.

I’ll report back when it's all back together and I do a test flight.... In the meantime I guess I'l just have to fly the wife's Cub

 

[Full Throttle]

In the meantime I guess I'l just have to fly the wife's Cub

I fly my wife's Tomahawk all the time. It's fun...

 

[SwimmingDragon518]

Just recently I started having RPM surges while taxing after a hot engine restart. This does not clear up by running up the power (maybe full power but I haven't tried that).
Things I've checked:

• Looked for intake leaks...nothing obvious
• Looked at the #2 exhaust valve for being sticky (it was sticky 26 hrs ago - fixed)
• Bottle tested the injectors, all good

Engine - IO-360-M1B, 250 hrs since new.

Here's an engine graph during the surging. One is with MAP/EGT and one with RPM/EGT
Here's a video of the engine analyzer during the surging...corresponds with the graphs.
View attachment 97838View attachment 97839

Hey Mike,

I had a somewhat similar issue as you a while ago (see link below), although it only happened when cold and wasn't so pronounced or rhythmic. I added some MMO in the gas and oil and all has been good since, ~ 8 months and 60 hours later. No idea what it was but I always assumed there was a tiny bit of FOD in the #2 exhaust valve that got cleared with the MMO.

I know there are 1 million opinions about MMO, but it literally stopped my fluctuations on the next flight.

Good luck!

Slightly fluctuating EGT and lagging CHT on startup on cylinder 2

Hello, I have a very subtle problem that I noticed the other day when I was in normalize mode at startup. It looks like EGT on #2 fluctuates a little and CHT lags a bit behind the rest of the cylinders. Can't feel much seat of the pants but if my tip up prop is resting slightly open on its...

vansairforce.net

 

[mfleming]

OK, Here's an update to the original post.

Avstar thought the flow divider might be the culprit. So I sent them the flow divider and fuel servo. They inspected everything and said they looked good and shouldn't have been the source of my troubles. Although they did say they made some minor tweaks. I received the parts back and ground ran the engine. No symptoms but I hadn't flown and gotten everything up to operating temperature.

Before I cowled the engine, I decided to pull the #2 intake tube and replace the 'O'-ring and gasket just in case Savvy Aviation was right about a possible intake leak.
I know I know, never make more than one change at a time but by this point I didn't want to cowl, than un-cowl the engine if the fuel injection inspection hadn't fixed anything. Well today I took it up for a test flight and the issue seems to have disappeared.
Here's a link to the uploaded data from today's flight, plus the hot restart after the flight. No surging and the #2 EGT didn't drop off like crazy at idle.
Flight 10/31/2025
Hot start and short run 10/31/2025

It looks like the surging and rough running are gone. Was it the small tweaks on the fuel injection system or the #2 intake tube 'O'-ring/gasket replacements?? Hard to tell but I think it was an intake leak.

 

[lr172]

OK, Here's an update to the original post.

Avstar thought the flow divider might be the culprit. So I sent them the flow divider and fuel servo. They inspected everything and said they looked good and shouldn't have been the source of my troubles. Although they did say they made some minor tweaks. I received the parts back and ground ran the engine. No symptoms but I hadn't flown and gotten everything up to operating temperature.

Before I cowled the engine, I decided to pull the #2 intake tube and replace the 'O'-ring and gasket just in case Savvy Aviation was right about a possible intake leak.
I know I know, never make more than one change at a time but by this point I didn't want to cowl, than un-cowl the engine if the fuel injection inspection hadn't fixed anything. Well today I took it up for a test flight and the issue seems to have disappeared.
Here's a link to the uploaded data from today's flight, plus the hot restart after the flight. No surging and the #2 EGT didn't drop off like crazy at idle.
Flight 10/31/2025
Hot start and short run 10/31/2025

It looks like the surging and rough running are gone. Was it the small tweaks on the fuel injection system or the #2 intake tube 'O'-ring/gasket replacements?? Hard to tell but I think it was an intake leak.

My guess is this resolved it:

"They inspected everything and said they looked good and shouldn't have been the source of my troubles. Although they did say they made some minor tweaks. "

Too me, that means we didn't polish the flow divider piston or it's bore as well as we should have on the production line, but prefer not to admit it. The fact that you could occassionaly get the problem to go away with a high power run points to the divider piston ocassionally getting hung up in the up position and not always coming back down (= rich idle/dieseling). They probably looked at it and said "that doesn't look that bad, but we should really polish that better." Viola, problem disappears.

Glad you got it resolved.