Runs Rich and Tach reads 2k or 0 regardless of RPM

[Ukko]

Howdy,

Long time lurker first time poster and all. I have a E30 that I picked up last year, long story short it is a project car on this particular project's third owner. The previous two owners rebuilt the engine and did a auto-manual swap. When I bought it it was still in engine grease, there was no prop shaft, and 100 other things were not finished. I have been working on getting all that finished. It is a mixture of a group of donor cars, the chassis is from an early '87 coupe, the engine is from a '90 with 80k on it when it was rebuilt, the engine wiring harness is from an '88 super eta, and the transmission is from some sort of 5 series.

I finally have been sorting out what I can and it is running and road worthy (to a degree) but it is not really right. Any help in diagnosing things would be a big help. Here are my symptoms:

1. Running rich (carbon in the exhasut and strong oder).
2. Misfires at high throttle.
3. Backfires after too much misfiring.
4. Tachometer reades about 2100 RPM reguardless of engine speed. Observed range is from 1900 to 2500.
5. When engine is coasting tachometer drops to 0.
6. The trip computer is reporting about 9 MPG.

I don't have any codes or CEL. Originally, I had an O2 sensor code because there was no O2 sensor, I installed one and that went away.

I think that the most telling symptom is the way the tachometer is acting. Originally, there was no tach signal at all, so I replaced the batteries. that did not work, after some searching I figured out that the problem is the the body harness expected the tach signal to be wired via a connector in the glove box while the engine harness was routing the signal through the C101 connector on the firewall. I rerouted those wires and now I am in my current situation.

I have tried two DMEs and two clusters with similar results.

My gut feeling is that the DME is either getting a bogus signal from somewhere or is in some sort of limp mode, but why am I not getting any codes? When costing the DME will shut off the injectors, then my tach reads 0. Shifting into neutral will bring the tach back up to 2k.

Any suggestions on what I should look at next?

Thanks!

[Ukko]

A follow up with more info. I checked the resistance on the position sensor and the thermostat, both were in the appropriate range.

I also swapped the wires I had rerouted from the C101 connector, on the chance I had messed them up. The was not helpful, both readings were obviously wrong.

Any guesses?

[stamar]

ok the tach

theres a youtube video that shows a screw that often comes loose.


have to take out the entire thing.

theres one for the gas guage and one for the tach one each side of the cluster.
it is related to the 8.8 mpg

the other stuff sorry no input.

[jlevie]

Go back and recheck your tach and econometer wiring. I suspect you have the signals swapped.

[Ukko]

Go back and recheck your tach and econometer wiring. I suspect you have the signals swapped.

I looked into this and swapped them just for giggles. It was worse in the swapped configuration. The econometer just sat in the middle and did not act normal or sane. I so I swapped them back.

I will pull the cluster and look into the screw stamar mentioned this weekend. Does anyone know what the tach signal is supposed to look like? I have an old school digital multimeter with a built in oscilloscope I could figure out how to use if I even knew what I was looking for. I assume it is pulse based, is it one per revolution? Is it tracking the firing of a single cylinder or all of them?

(I am not an electrical engineer, but I got a good deal on a used Extech multimeter a long time ago. That is like saying I stayed in a Holliday Inn Express or whatever.)

Thanks for all the help!

[jlevie]

The tach signal is one pulse per rev and will resemble a square wave.

[Ukko]

Well, I have not made any progress on the cluster the fuel pump gave out and I spent the weekend replacing that. After that was sorted out the car ran much better, although it is still rich. Obviously there is more than one thing going on here.

Since I replaced the fuel pump it has died on me twice. In both cases it was after running for a little while, 10-15 minutes, basically the engine acted like it was not getting any fuel. I could get it to hobble along for a bit at 5-10 mph in fits and starts but hen it died. After turning off the ignition and restarting it came right back and we were on our way as if nothing had happened.

The last time I did get a CEL, I checked the code with my Peak reader and it came back as 0A which is the bad lambda emissions code.

My relays, and O2 sensor are all new, so any other guesses. I am thinking it might be the in-tank pump becasue it feels kind of like a fuel problem, and that is about the last thing to change. But why would it come back so easily after stalling out. I just don't know the failure modes of that part.

jlevie Thanks! I will get the scope on there and see what I am getting. Do you know how long the pulses are relative to each other? That is does it look like a textbook square wave or more like a series of spikes that just happen to be square?