Chris Lockhart

'89 S4 5 speed supermodel. Started yesterday. Drove the car at lunch, all is well. Go to leave work, and after a few minutes it starts stumbling/misfiring (or whatever you want to call it) This afternoon I checked plugs/wires, no obvious defects. (I also plan to do a check in the dark to see if I see any sparks) I started it up, and started listening to the injectors with a stethoscope and it was running perfectly. No stumble whatsoever. I checked several connections anyway. Temp sensors, etc... (trying to remember my 928 stuff, it has been 4+ years since my '89 GT.) and basically about 5 minutes in, once it warmed up, it started stumbling/misfiring. VERY obvious. Given the limited info provided, any and all help in guiding me in the proper direction is appreciated.

Chris Lockhart

AO

You can try pulling each lead to the spark plug one-by-one to see if you can isolate it. Then it comes down to spark or fuel. Stick a spare spark plug in the lead to see if you have good spark. If you do, try pulling the injector lead to see if it changes.

Mrmerlin

I would check the coil wires first see,
if they are rubbing on any part of their run, if so thats an issue

also remove them and inspect the ends for corrosion

Next remove the vacuum lines from the dampers and the FPR see if you smell gas if you do then that part needs to be replaced

Chris Lockhart

Andrew, I thought that pulling the wires was VERY bad on a 928? Between high voltage/amperage zap potential, and the potential to fry the EZK if there is not a good path for the energy to pass to ground.

Will one of those "inline" spark testers work?

Merlin, I pulled the drivers side coil wire off last night because I didn't like the way it was routed. It was in a bind and stretched to it's limit, plus was not seated all the way in the coil. (it was only about halfway in) I rerouted it where it's not stretched outlike it was.

I'm trying to figure out why it starts acting up after it gets warm. What is the difference with the fuel delivery at the change over point. Doesn't the computer have like a "cold start" map or something, that controls warm up until the o2 sensor comes up to temp? Could the o2 sensor be bad?

I listened to the injectors and they all sounded the same before and after it started missing.

Thanks guys.

AO

Well I wouldn't go driving the 928 with a spark plug or two disconnected, but at idle I have done this several times with no problems. What you're trying to do is see if the miss gets worse or stays the same.

Once you identify which cylinder(s) are suspect, then you need to look at the condition of the wires by hooking up a spare spark plug or an in-line tester - but I prefer an old spark plug so you can see how strong the spark is. If the spark is weak, unscrew the spark plug connector from the wire and check the condition of the wire. I have seen corrosion at the connector on may wire sets. If it looks good and Ohm's out, then I'd look at the injector.

The injectors can sound okay, but still be plugged up. I've whitnessed this first hand. The injector was clogged. If you energized it, it clicked, but nothing (or very little) was coming out.

So you do the same test. With the engine running, you pull the injector leads off one at a time to see if the car's idles degrades. By now you should have an idea which cylinders are suspect. If the idles stays the same... Ding ding ding. you have found the culprit.

As for acting up when it gets warm... Yes the LH has a warm-up map that it overlays on top of the base map to enrich the mixture. It could be in your case, this enriched mixture is enough to mask the issue of a compromised injector. You can check the Temp II sensor readings, but it sounds like that's working. The other thing that could be an issue is the O2 sensor. If the O2 sensor is falsely reading the mixture as rich it will try to lean the engine out - again uncovering other issues.

One other thought. Caps and rotors, but I doubt it.

Mongo

If it does it when warm, it could be the LH going bad. John Speake had mentioned that this was another symptom of failure. The car would/could misfire, run overly rich, and have an erratic idle.

Do you have the WSM? There is a guide for troubleshooting the LH. I see your car is an '89 also. Can you borrow a Spanner or Hammer from someone?

cpayne

+1 on Merlins advice. I had similar issue on my car. Coil wires kept migrating toward other metal along its path and sometimes ran great other times stumbling. Arching is very apparent in the dark. A faint Blue strobe in my case. Landseer told me I should put some type of rubber insulating hose around the wires. I think I even saw a post recently in another thread with the wires insulated even further.

Lizard928

Instead of pulling the ign lead, I pull the plug off the injector.
It's safer and then you aren't putting fuel into the cyl that isn't firing 100%.

Landseer

I'd pull the distributor caps and check the condition of the electrodes if you haven't done it yet.
Start simple.

John Speake

When you start the car from cold the LH runs open loop (ignores O2 sensor), When temp 2 water temp get to 60-80 degC (varies with software version) the LH changes o closed loop.

You could try disconnecting the O2 sensor with the battery ground strap disconencted, that will force the LH to recognise no O2 sensor is present. Reconnect battery grouund and see if the car runs better.

Does car stumble at idle as well as when on the move ?

Chris Lockhart

Yes John, about 5 minutes after a cold start the engine starts to miss whether driving or idling.

auzivision

I had a bizzar intermeitant mis that went away when I had my MAF rebuilt... just another idea to through on your radar screen.

dprantl

From the symptoms you describe, I would check the O2 sensor and its wiring first.

Dan
'91 928GT S/C 475hp/460lb.ft

mikkol

When You check the distributor caps, remove the plastic black covers over them and check if there are any signs of burning. While I was tracking misfiring, I also cleaned my MAF (electrical solvent, isoprophyl alcohol spray bottle, I think) and cleaned grounding points , especially one under passenger side coil.

Chris Lockhart

Thanks for all the input guys.

Dan/John, because of the fact that it starts acting up after it warms up, I'm really inclined to start looking at the o2 sensor and it's connection. It sure seems like it starts after the o2 sensor has started sending it's signal to the LH. 3 or 4 minutes after startup. Smooth as glass until then.

That being said, I am ruling nothing out until it's found/repaired.