What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RPM Drop/Low CHT & EGT

GrinchF16

GrinchF16

Well Known Member
RV7A
O360
1xMag 1xElectronic Ignition
Archaic GRT EIS

Beautiful day to fly. Sitting at EOR, do standard run up and mag check. When I switch to Mag, RPM drops off very rapidly. Back to Both or Electronic Ignition, runs fine. As I cycle through EIS, I see that EGT/CHT numbers on #1 are both significantly below other cylinders regardless of Mag/Electronic Ignition selection.

One A&P says bad Mag, one says check plugs and wires. I understand the logic of both of those but how do you explain away the temps? Seems to me temps indicate not much burning going on yet engine seems fine with Electronic Ignition selected.

Mea culpa - I’m a decent pilot, the mechanics side is new to me.
 
Last edited:
Ironically I had a very similar experience 3 months after owning my RV. Ran perfect up until one mag check.

Ran great on the P-Mag and shuttered on the Bendix. CHT/EGTs were not mapping like they should.

Here is what I did/found. Working from the easiest / least expensive fix first.

*Visual inspection > Found the P-Mag lead to Cylinder 1 (bottom) resting on the plug. Visible arching on the plug tip which led me to believe this had been that way before I pulled the cowl.

*Tested the Left / Right path from the key switch > Finding the P-Mag with a loose wire led me to believe that my L/R may be backwards and I should be looking at a P-Mag fault not the Bendix. Wiring was confirmed correct. The plane was stuttering running on the Bendix.

*Visual inspection of the plugs > Looked normal for 20 hours post cleaning. Minor footballing on the aviation plugs told me they had some miles on them. Gaps were set the same on both the aviation and auto plugs.

*Visual test of the spark from the Bendix as the prop was manually rotated. Strong spark on all 4 leads. (300 hrs since rebuild).

*After cleaning the aviation plugs (champion). Tested with normal voltmeter. Resistance failed on all 4. Two were completely open.

*Replaced all 4 aviation plugs and 4 auto plugs. Gapped the auto plugs to the P-mag 114 specs and the aviation plugs to tempest specs.

Run up was back to normal and the CHT/EGTs were what I would expect running on each mag separately. I have no idea how the engine ran so smooth on the P-Mag. I am guessing the low CHT/EGT on cylinder 1 was due to the lose wire when on the P-mag.

It had quite a few of us at the airport scratching our heads on Sat morning when this happened.

I hope this helps with your troubleshooting.
 
GrinchF16 said:
As I cycle through EIS, I see that EGT/CHT numbers on #1 are both significantly below other cylinders regardless of Mag/Electronic Ignition selection.
Click to expand...

If that is true, it is not likely an ignition issue. With one plug firing instead of two, EGT should rise over that with 2 plugs firing. If both plugs stop firing, EGT goes to 0; pretty easy to spot. Generally speaking, ignition issues result in high EGTs or 0 EGT. Low EGT is more related to air fuel issues and compression issues, excluding wonky timing issues which rarely appear on just one cyl. If you have consistently low EGT/CHT on one cylinder, you are looking for a reason for low power output on that cylinder. At idle level RPMs, an intake leak will do this (will not do it at WOT), as will a loss in compression (numerous reasons for that).

I would start with inspecting the intake for leaks and do a compression test on #1.

Larry
 
Last edited:
Stuck exhaust valve

As have been said, low temperatures indicate no power on the #1 cylinder.
A compression test or remove rocker box cover plate on cylinder head #1.
Check before turning prop that ignition is OFF.
Turn propeller by hand and check movement on the exhaust valve.
If exhaust valve is stuck You need to remove the valve and ream the guide.
There is plenty of information at VAF.
You do not need to remove the cylinder to do this.

https://www.lycoming.com/sites/defa... Reduce the Possibility of Valve Sticking.pdf

Good luck
 
Thanks

Compression check is on the books.

The EI plugs have less than ten hours since new.

I’d much rather fly…
 
No power

Low CHT/ EGT temperature tells us that there is no power on cyl #1.
If compression is OK, then there is no ignition or no fuel.
As I understand it you have a carb engine. This implies no spark.
Check for visible spark. Be sure to have the spark plug grounded at all times.
Otherwise EI can be damaged.

Good luck
 
Another easy check

Something else to consider is an intake leak on that cylinder. A small leak would have the cylinder running lean so you would expect higher temps but a larger leak would mean large reduction in power from that cylinder and might give you the lower temps you are seeing. Check both the intake gasket and the tube where the intake manifold leaves the sump.
 
You must log in or register to reply here.
Back
Top