bonus12

Hello and thanks for any help. This is for a 1986 944 turbo. The engine is missing some beats at idle. I can especially tell it's missing when I listen to the exhaust at the tailpipe. The idle is pretty constant. The car runs really well -- this problem is only at idle. How many misses are normal?

Here's the thing. The idle is perfect when the O2 sensor is unplugged at the back of the engine. When I plug it back in with the engine running, it continues to idle perfect for a few moments until the problem comes back. Interestingly, if I turn off the engine and turn it back on, the idle is perfect for a few moments until my a/f gauge starts reading and the missing/roughness returns.

I replaced the O2 sensor but the problem isn't fixed. I've also changed the spark plugs the other day and have done a full tune-up recently, including getting the injectors professionally cleaned and tested. I don't think there are any vacuum leaks. Vacuum, according to a gauge I have, reads about 16-17 psi when the engine is warm. I've also replaced many hoses under the intake.

I am wondering if it's the speed and/or reference sensors, which both have broken plastic connectors. Could this be the issue?
As for the ICV, The problem does not change when I unplug it. Also, the ICV vibrates with the ignition on and the engine off. Could it be the ICV?

Please let me know what you can suggest. I will try to implement ideas and will report back. Thank you!

Dwizle

Sounds like a issue with theO2 sensor side of the DME it runs fine on the DME tables but when the O2 sensor starts making adjustment things go hay wire try checking the wiring from the plug in the engine bay to the DME for shorts with the DME disconnected also try unplugging the sensor and reading the voltage the single pin to ground while the engine is running and after it's warmed up. see if it fluctuates between 0-1 v then hooking it up and back probing the DME plug and see what the voltage is there.
If they are the same you have a issue with your DME

jimbo1111

Its best if you can provide information on your car first. Is your car stock or modified?

bonus12

Thank you for the replies so far. The car is stock except for Autothority stage II chips (on the DME and KLR). It has the restricted banjo bolt but of course that is not in play during idle. I suppose I should put the stock chips in and see what happens.

I was wiggling one of my shot reference sensors (in fact it was the speed reference sensor) last night. It was extremely sensitive / loose. But now it seems to be a little more locked in / tight. That seemed to help. This morning I was idling and it's much improved. I had the intake off earlier this month so it makes sense that I could have messed up the delicate reference sensor connections. But I am still wondering how likely this would be to cause an idle that smoothed out when the O2 sensor was unplugged.

To be honest, the idle was not bad to begin with. It was just some misses here and there, but enough to get my attention. Maybe I am misguided to chase down the perfect idle? It sounds like the advice I am receiving is to look into engine management at the DME. I have a DME from a NA of the same year. Pelican parts seems to indicate that it's the same unit, but I'm not sure I can swap it into my turbo for testing purposes.

When I get a moment I'll go ahead and check to see if the O2 is delivering its voltage to the DME.

jimbo1111

I would start with the basics. Like plugs, wires and fuel pressure before heading toward the dme. When was the last time you had a tune up?

bonus12

Thanks for replying. I went for a drive today and, although it's much improved, there is still some missing during idle. It's pretty hard to tell it's there unless you're paying attention.

Just changed the plugs. They are brand new and gapped.
I switched out the plug wires with a backup set I had and there's no change that I can tell.

I haven't measured fuel pressure, but I did order a 3 BAR which is coming soon. I was not planning on using it until I got a wideband a/f gauge and O2 sensor. But I could use it to help me diagnose this issue. Otherwise, I do have a backup stock FPR I could use. Is there a reason it would go bad?

Fuel filter, air filter, Cap, and rotor were replaced 16,000 miles ago. Dist cap does have some minor pitting -- I could try another one I have on hand, but am I wrong to think that things like wires, caps, and rotors would cause a problem that would not go away when unplugging the O2 sensor?

Another clue is that the symptoms go away temporarily when the engine is turned off and then on again. I know about the closed-loop and open-loop. Just trying to figure out how it's all interacting.

Thanks again!

thomasmryan

by pass your wideband A/F and see if the closed loop miss occurs.

it would be wise to replace the speed and reference plugs. the output voltage goes up with speed but if it was too low, open loop would also suffer.

bonus12

Ok, I'll replace those reference sensors. Oh, and I currently only have the stock O2 sensor and a narrowband gauge -- no wideband yet. I'll also mess around a bit with the spark plug wires, maybe there is something there.

thomasmryan

How fast does the O2 sensor switch? See if it switches at the same rate with your monitor hooked up.

Open up the hood/bonnet in a dark area. You might see the open part in the plug wires sheathing /insulation

bonus12

Haven't seen any sparks jumping or anything like that in the dark but I'll look again tonight and maybe spray some water on them. I've also read that a spark plug wire should have between 10,000 and 12,000 ohms of resistance per foot. I'll confirm that # and measure them.

Not sure what you mean by the monitor? I could measure the O2 sensor voltage output with a multimeter today and see how fast it switches. Since it's a brand new one I hope it's acting normally.

The help is much appreciated!

shortyboy

I would definitely replace speed/reference sensors and harness too. Have you tried changing DME FQS settings to see if it helps? Do you have wideband O2 gauge to see what your AFRs are?

bonus12

I've checked my FQS setting only to verify that it was at position 0. Would changing it not be like covering up the problem?

Going to order the reference sensors tonight -- should be here next week. And I currently only have a narrowband gauge.

Today I tested the resistance of all the spark plug wires. I was reading online that domestic plugs and euro plugs can have different resistances. Anyway, out of the 8 wires I tested, all were 2.6 - 3.0 k Ohms. They were all very consistent. No apparent damage on them, either. So I can't see that being the problem. The coil plug had basically 0 resistance.

I replaced the cap and rotor with the backup parts I had -- no changes. I even replaced an intake manifold gasket, and my vacuum psi might have improved a fraction of a lb, but I still have some missing at idle.

I also rented a compression tester from my local auto parts store, but unfortunately it ended up being broken when I went to use it. I wanted to rule out a leaky valve or ring. However, the problem is only at idle.

Next step is those ref sensors. Thanks for all the help. I would love to get this fixed.

bonus12

As for the reference sensor harnesses, when is replacement recommended? My harness connectors are in good shape.

bonus12

I forgot to mention something. I tried unplugging the O2 sensor again during idle. Yes, it smoothed out but when I revved the engine, it stumbled and fumbled a little bit.

-- O2 sensor plugged in: engine revs perfectly, but idle misses a bit
-- O2 sensor unplugged: idles perfectly, but misses when revving

How can this be?

MAGK944

You haven’t eliminated a problem with the ICV by simply unplugging it. The ICV could be mechanically stuck thereby preventing it from providing the required bypass at idle and causing your misfire. When it vibrates with the engine off does the voltage drop on battery gauge? That could be a sign of the servo motor fighting a stuck valve.

Not saying that is definitively your problem, just that you’ve not eliminated it as a possible cause.