Manifold Pressure Mystery

[Tango Mike]

Manifold Pressure Mystery:

The airplane: RV-4 with VM-1000 and dual Electroair ignitions (left gold Gen 1 unit with the MP sensor mounted on the EIS beside the coil packs, and right purple Gen 2 unit with a rev 1 MP sensor mounted separately).

Symptoms:

1) VM-1000 MP reads barometric pressure when the engine isn?t running.
2) After start, the MP advances to mid to high 20s with about 900 rpm.
3) Left advance reads 17 degrees
4) Right advance reads 28 degrees
5) Neither advance reading changes when the manifold advance is turned off.
6) During runup, the VM-1000 MP and the advance on each side remain the same as when the rpm is below 1000.

Discussion:

1) It appears to me that the VM-1000 and both Electroair EISs are operating normally.
2) It?s important to note that the Gen 1 units have a different RPM advance curve than the Gen 2 units. I don?t have my notes with me at the house this evening, but I think the Gen 1 units are programmed to 18 degrees once RPM is above 800, and the Gen 2 units are programmed to 25 degrees. (I also think the Gen 1 units added a degree or two of advance for every 100 RPM above 1000, but that?s not germane to this problem.)
3) With MP advance turned on and working on both sides, the advance readings at low taxi manifold pressures should be high. They?re not, and they remain at the fixed advance settings based on RPM, close to the programmed, fixed advance of 18 degrees on the left and 25 on the right.
4) Turning off the manifold pressure advance causes no change in advance readings.
5) If the VM-1000 were reading normally, I?d troubleshoot the manifold pressure ignition advance system.
6) If the manifold pressure ignition advance system were working properly, I?d investigate the VM-1000. (It?s important to note that last year I followed forum member Reggie?s procedures for cleaning the MPT and solved a problem with erroneous MP readings. That and the fact that the MP reads atmospheric pressure in static conditions appears to eliminate the VM-1000 as a suspect in this mystery.)
7) I?ve removed the hose from the fitting on the engine and gently applied pressure and vacuum, both of which resulted in about .2? increase and decrease, respectively.

What?s next?

Investigate the possibility that no MP is being supplied through the fitting on the #3 cylinder to the VM-1000 MPT and the two MP sensors?

Any suggestions would be much appreciated. Thanks.

Tosh

[rvmills]

Tosh,

I have a later model Electroair EI and a VM-1000. My system has a separate MP hose to the EI MAP sensor (not teed into the MP line to the VM's MP sender…kinda sounds like yours may be combined?). I also have a mini-toggle to turn the EI MAP sensor on/off. Yours may be different, but things I'm thinking to check are:

- EI MAP sensor (unless your gen EI takes it from the VM-1000).

- EI MAP sensor on/off switch and its wiring (at the switch, and all the way to the sensor, and to the brain box).

- Tubing between engine and MAP sensor, if it is separate from the one you applied suction and pressure to (seems like it would have to be kinked or fully blocked to have this effect).

My gut says its trons…but that's a SWAG.

Good luck!

Cheers,
Bob

PS: Reggie's a good guy on the VM stuff, eh!

[Tango Mike]

Bob,

The routing is: engine to VM-1000 MPT sensor, which has a 90-degree angle fitting connected to a T-fitting that supplies each MAP sensor separately.

I've examined all the tubing as best I can without removing it so far, and find no evidence that it's compromised.

The tube from the engine to the VM-1000 MPT is relatively flexible and runs across to the MPT with cable ties securing it to the engine mount and other fixed places, one of which appeared to have squeezed the tubing a bit. I removed that tube and tested to make sure it wasn't blocked.

To eliminate the possibility that it might somehow be the case with the engine running, I used a length of stiff rubber hose from the engine to the MPT sensor and ran the engine. Nothing changed.

I haven't yet considered a wiring problem within the EI circuitry because I'm not yet convinced this is an EI system malfunction.

If the VM-1000 didn't read atmospheric pressure without the engine running and did indicate normal manifold pressures at idle and run-up RPM, I'd be able to eliminate the supply of MP from the engine as a factor and concentrate on the EI.

I'm wondering if it might be worth coming up with a way to test the manifold pressure with a gauge connected to the tube from the engine and start it. If I see normal idle manifold pressure under that condition, I can at least know that it's being supplied as far as the fitting on the back of the MPT.

Tosh

[Pat Hatch]

Tosh,

Although it seems highly unlikely, it would appear that both of your MAP sensors are inop. Is there anything common to both? Like an excitation voltage? I would be tempted to replace one of them or find a loaner MAP sensor and swap them out and chase it down that way.

Hope this helps!

[az_gila]

Originally Posted by Tango Mike

Manifold Pressure Mystery:

....
7) I?ve removed the hose from the fitting on the engine and gently applied pressure and vacuum, both of which resulted in about .2? increase and decrease, respectively.
....
Tosh

That's a pretty small value for a test.

Try a 27 inch water difference in a tube manometer and look for a 2 inch reading change.

[Tango Mike]

Pat,

I agree that it's unlikely, but even if it is true, that can't explain the failure of the VM-1000 reading.

The more I ponder this and get suggestions, especially Gil's noting that my test isn't a very good one, I'm tempted to focus on the only item other than the engine manifold pressure line that is common to both the VM-1000 and the EI, which is the MPT.

If it's allowing atmospheric pressure to read normal, but then malfunctions as soon as engine manifold pressure is applied, could that be an internal failure in the MPT that affects both the VM-100- reading and the ability of the EI MAP sensors to supply MAP advance to both EIs?

[celstar]

Tosh, I believe you mentioned that there was a ruff engine on run up with one side off? Also what is anyones thoughts on a possible induction leak?

[Tango Mike]

Originally, I thought that other than manifold pressure from the engine, the only component common to the VM-1000 and both EIs is the MPT on the cockpit side of the firewall. Within the last year or so I had refurbished it and the possibility that it had failed in some way so quickly didn't seem likely.

The only consideration that didn't make sense was the question of how a component down line from where the EIs get the manifold pressure could affect how they get it and screw up the manifold advance.

Well, duh. The MPT isn't common to both the VM-1000 and the EIs.

But four other components are, and at this point I am totally baffled as to how the system could have worked before and doesn't now.

The image included with this post shows four fittings and a diagram of how they were installed in the airplane. The MPT has female pipe threads that accept the male threads of the 90-degree fitting closest to the firewall. The nipple on this fitting feeds manifold pressure to a T-fitting (not shown) that supplies both EI MAP sensors.

The fitting labeled "EA" has a plug inside with the letter "A" inscribed on it, and the word "FLOW" inscribed on the side with an arrow pointing toward the engine.

The nipple attached to the EA fitting receives manifold pressure from the engine, but it enters it in the opposite direction from the arrow.

There's no other way these four fittings could have been installed except to put the EA fitting closest to the firewall, which would still leave the FLOW arrow pointing toward the engine.

Can anyone make sense of this for me?

[az_gila]

There is no "flow" involved.

I think what you show is simply a "T" made up of multiple different sized fittings.

Taking it apart, cleaning the threads and adding a thread sealant would probably be a good idea...

The fitting connecting to the EIS sensors looks rather small. Is it correct for the tubing you use?

[Tango Mike]

Thanks, Gil, I have to disagree.

Unlike the other three fittings, which you can blow through and see light from one end to the other, the fitting labeled with EA, FLOW, and the arrow has what looks like a solid plug inside. No light passes from one opening to the other, and if I blow on either end, I can't get any air to move through the fitting.

It's a different fitting altogether. I think it might function as a "snubber" that prevents full, unregulated manifold pressure from the engine entering the MPT on the cockpit side of the firewall. It allows pressure variations to pass through the "plug", but slowly so as not to damage the MPT.

The tubing isn't exactly all the same size, but my understanding is that pressure variations don't care. Pressure and vacuum alike fill all the space they can reach.