Noisy Starboard transmission...rich engine

[Viper]

Qr Bbpost I'm beginning to suspect a vacuum leak somewhere... that would explain rough idle and the IAC being quite different between the two engines at idle. You do need to figure out what's going on with fuel pressure though (per last post)....

Agreed

[km1125]

I think you're pulling stuff apart too early!! Cleaning the injectors might not hurt anything, but just wastes time while you're trying to figure out what the ACTUAL problem is!

Look at this part of the readout:

carver_IAC_edited.jpg

The red circle is telling you that the engine is LEAN. It is also telling you that the engine is in "Closed Loop", which usually means that you do have an oxygen sensor (even if there is no catalytic converter involved). Right above that it shows "oxygen sensor voltage" is 0.00 mV. Is there an O2 sensor on this engine?

This ALSO shows you that the IAC is FULLY CLOSED (IAC position matches base position). It also shows that the "Desired IAC position is also at the base position, meaning that the COMPUTER is telling the IAC to close all the way, which means that air is getting into the manifold SOME OTHER WAY (like a gasket leak or missing hose). Need to find that.

[Viper]

Post your engine serial number so we can check exactly what you have.

[bud37]

km1125, viper......,I am trying to learn .....I would like to ask a question?, don't want to sidetrack....is it possible something could fool the ecu into thinking the engine is lean when it may not be and then over fuel it creating this issue.....

[405driver]

I'm trying to learn also...
It will be late tonight before I can post anything...out to dinner and a play before this drives me nuts...

[km1125]

Qr Bbpost is it possible something could fool the ecu into thinking the engine is lean when it may not be and then over fuel it creating this issue.....

Yes, that's certainly possible and something you need to consider while troubleshooting. A symptom could be a cause and not an effect or could be an effect by a completely (seemingly) unrelated cause. Something like a bad (or disconnected) coolant or air temp sensor could do that. Also, something like a bad O2 sensor could cause the engine do do just that. The O2 sensor is really the only thing that tells the ECU if the engine is lean or rich, otherwise the ECU is just "guessing" based on all the other inputs.

However, with the IAC also commanded down low that means there is plenty of air (too much) getting in the engine to run it at idle, even though the IAC is commanded shut.

The fuel pressure would definitely cause a rich condition too, even though the ECU isn't intentionally trying to make things richer. The ECU is commanding the injectors with a "normal" pulse, but with a higher-than-correct fuel rail pressure, the injector would squirt more fuel than the ECU is expecting.

[km1125]

And overall, in troubleshooting issues, Viper's comment of "We can throw out all kinds of theories here but we need more accurate symptoms or it's just a guess at best." is all too true. There can be 1000 things not right with that engine but if we're trying to find out what is causing an unstable idle we need to stay focused on what the exact symptoms and measurements are and what makes sense that would cause that issue.

I could come up with 100 theories of things that just might just possibly maybe have an issue that would cause it and start running those down, but it's a WHOLE LOT more work and end up being a livewell full of red herrings. But I'll give you an example for fun... suppose there is a slight blockage in the hydraulic shift body on the transmission and that is causing both the fwd and rev clutches to slightly engage at idle... causing too much load on the engine, which the ECU can't compensate for and that's causing the noisy transmission. We could do lots of looking into that... but DON'T, because it's HIGHLY UNLIKELY at this point and DOES NOT match some of the OTHER GOOD DATA that we have.

[km1125]

One more thought... and it's related to the IAC being commanded shut:

You should check and compare the throttle plate on the throttle body to see if it is going to "0". I'm not sure on that body if there is an adjustment just like the old throttle idle adjustment on a carb. You could also compare with the other engine.

If the throttle plate isn't returning to closed all the way then it's letting in too much air and the IAC has to close more than it should at idle to try to compensate.

In my mind, you have two KNOWN issues that should run down before moving on to others:
1) Fuel pressure at idle should be ~30, but you're reading 37-40 (or at least you were the 2nd time around). This could have definately been caused by the manifold vacuum line being disconnected but if that's been reconnected and you still have the 37-40 PSI reading at idle, then something else is wrong... either the vacuum hose is leaking, disconnected on the other end, or the fuel pressure regulator is having a problem. btw, the disconnected/leaking hose would also contribute to #2, the IAC problem.
2) IAC is being commanded to minimum position at idle.

[405driver]

Gee... thanks for all of that. At least with the top off the engine, I can look at all of the vacuum hoses. It was obvious that the plenum ( or whatever it's called) has never been off so I don't think that having the injectors looked at is a bad idea. ,Also replacing the FPR seems reasonable seeing as the FP was fluctuating quite a bit.
Thankfully, it isn't the compression. So, there is an answer. With everyone's help, I'll keep looking.

[Viper]

Qr Bbpost

Qr Bbpost is it possible something could fool the ecu into thinking the engine is lean when it may not be and then over fuel it creating this issue.....

Yes, that's certainly possible and something you need to consider while troubleshooting. A symptom could be a cause and not an effect or could be an effect by a completely (seemingly) unrelated cause. Something like a bad (or disconnected) coolant or air temp sensor could do that. Also, something like a bad O2 sensor could cause the engine do do just that. The O2 sensor is really the only thing that tells the ECU if the engine is lean or rich, otherwise the ECU is just "guessing" based on all the other inputs....

++1
The ECU when functioning properly, simply does what it's designed to do which is deliver the correct amount of fuel based on sensor input. Good data, or bad for what ever reason, the ECU will react accordingly. It'll either deliver fuel based on what it assumes to be correct data or set a default if it has reason to believe the data is faulty such as a case where a sensor is obviously out of range or the circuit is hi/low (sensor circuit is open or shorted) when it shouldn't be.