Open Road

Been working on this issue now for a month or so and have no clue as to where to look next.

1988 S4 Auto with 46,000 miles.
I had recently completed a top end refresh - car ran great except for a random (not every time) no hot start. It was suggested that I send the MAF and LH out for refurb. Whilst they were away, I decided to remove the electrical panel, replaced the relays and clean up all the contacts. I re-installed the rebuilt LH and MAF and now the car is not running well at all. It has a pretty low idle as well as a really weird popping/sucking sound that comes and goes whilst idling as well as soon as I shut off the ignition. See the attached video. It is also misfireing/stuttering once it gets on the freeway. Speedtoys came over yesterday with his parts car and we switched over both the MAF and LH - neither of which made any difference. My MAF and LH worked just fine when installed on his car.

Everything new as follows
LH
MAF - Also replaced the 5 Pin connector
TPS
Temp2 - also a new connector
Idle Control Valve - not replaced but tested good with a 9V battery.
All New Gaskets on top end
Spark Plugs & Wires
Fuel Dampers and Regulator - only 5 years old and all hold vacuum

Would love to hear everyone's thoughts on the weird noise. Any Ideas ?

You posted this yesterday to TOS verbatim.

No video post here.

When you did the intake refresh, did you mess with the LH harness grounds? These are the ones bolted to standoffs on the rear of the block.

Did you do a smoke test after you finished the refresh? Or, while attempting to diagnose?

Let me get this straight:
Can ran fine before intake refresh.
After intake refresh it exhibited no-start when engine at operating temp.
LH, MAS rebuilt. CE panel removed and cleaned, *all* relays replaced (all $2k worth of relays).
Now car runs much worse.

my video is apparently the wrong format. what should it be ?
Car ran fine before refresh. Once refresh was complete it also ran fine but did have an occasional no hot start. I then sent the MAF and LH off to Roger. Whilst they were away I removed and cleaned the CE panel. I replaced all the 53 relays - non of the others. Car now does not run very well, feels like it is misfiring when you put you foot down. The idle is kinda ok but as mentioned there is a really weird, popping sound (that comes and goes) coming from what seems like inside the intake (only at idle and also as soon as you shut off the power with the key)

Giddyupp wrote: ↑Sat Jul 06, 2024 12:49 pm … there is a really weird, popping sound (that comes and goes) coming from what seems like inside the intake (only at idle and also as soon as you shut off the power with the key)

Do an intake smoke test. You can get a cheap smoke tester kit on e-bay.

Or, pay a shop to do it. But, they’ll probably charge more than e-bay kit costs.

just ran the smoke test and only possible leak I could see was the throttle linkage

Giddyupp wrote: ↑Sun Jul 07, 2024 8:52 pm just ran the smoke test and only possible leak I could see was the throttle linkage

Ok. Good. Now, back to another question I asked above:

When you did the intake refresh, did you mess with the LH harness grounds? These are the ones bolted to standoffs on the rear of the block.

Giddyupp wrote: ↑Sat Jul 06, 2024 11:14 am It has a pretty low idle as well as a really weird popping/sucking sound that comes and goes whilst idling as well as soon as I shut off the ignition. See the attached video. It is also misfireing/stuttering once it gets on the freeway.
…
Would love to hear everyone's thoughts on the weird noise. Any Ideas ?

I listened/watched the video on TOS. I can’t be sure since the sounds are not coming directly to my ears, but I think the popping sound is a no-spark misfire.

Given “ misfireing/stuttering once it gets on the freeway” I think chasing misfire is where I’d start.

You put in new plug wires. New wire sets have two different sets of coil wires. You use one set or the other depending upon which model coils you have. Look in the business-end of a coil: that’s where the difference lies. The distributor-side of the coil wires is the same. The coil-end is different between the sets.

Make sure the coil wires are fully plugged in and that you’re using the right set. Make sure the plug wires aren’t crossed-up. Make sure the plug-end is pushed securely into the plug wells. The weather boot should be 2-ish mm “pushed in” beneath the level of the cam cover.

I know those plug wires are NOT pushed all the way in. I don't know why but it didn't seem right that they should go down below the level of the valve cover. The coil wires and the leads are all correct - I have checked and double check them. Honestly Im so frustrated with this right now as I keep checking everything Ive done and cannot find anything wrong. I even pulled the electrical panel again yesterday and rechecked everything in there. If this is the issue Im gonna be so HAPPY - it's the one thing that does now make sense. Literally nothing else has changed (that I can think of). Unfortunately I won't be back at the car for at least a couple of days. I will report back once Ive checked them all. Im assuming I should push them in until they bottom out, could you please confirm.

Giddyupp wrote: ↑Mon Jul 08, 2024 2:02 pm Im assuming I should push them in until they bottom out, could you please confirm.

You should, literally, hear a “click” when the plug boots are properly seated. Absent that, the top of weather boot should be below the level of the cam cover.

Giddyupp wrote: ↑Mon Jul 08, 2024 2:02 pm I have checked and double check them. Honestly Im so frustrated with this right now as I keep checking everything Ive done and cannot find anything wrong.

Believe me: I get it. And it’s sometimes so frustrating that little things escape.

Check that all the plugs are torqued to spec. (I have found “finger tight” plugs in 928s brought to me.)

The belt covers have numbers on them that correspond to the cylinder numbers and are roughly in line with the terminals on the distributor caps
Beru plug wires are numbered.
I have found 928s brought to me where the right wires when to the right cylinders but 6/7 or 2/3 were swapped at the caps.

Last, take notes about the things you double check and data from tests you might do that generate numbers (e.g. volts, etc.) so that you can review rather than remember.

worf wrote: ↑Mon Jul 08, 2024 4:09 pm Last, take notes about the things you double check and data from tests you might do that generate numbers (e.g. volts, etc.) so that you can review rather than remember.

And maybe post those numbers in this thread. There are folks here that are wizards with these cars (you have one of them helping you, above) and will be able to tell you much more about what to check, once they have more, systematic data.

Cheers

Fnally got a few hours on the car again yesterday. Pulled all the plugs (which are new) checked the gaps (.028") and torqued them all to 18Ftlbs. Though the plugs are new they have been run on several test runs for probably at least 10 miles yet they all looked absolutely spotless. (not sure if this is an indication of anything).



Next I installed all the leads, confirming they are pushed firmly down onto each plug per the photo above. Then checked them all the way back to each distributor to confirm correct firing order. They are new Beru leads with the cylinder numbers right on them so it's kind of easy to confirm. I then pulled both leads off the coils, confirmed I was using the correct version and re installed making sure they are pushed all the way in on both ends of each coil.

Once complete I started the car. It did seem like it was running a little smoother but unfortunately im still getting the poping sound and the really low idle. I took it out for a drive and it did seem to run fairly well. there was a weird surge if I kept the rpm around 1800-2000 but if I put my foot down, there was no noticeable hesitation and it seemed like had plenty of power.

So I still have the low idle and the popping sounds (This is only at low idle and does go away as soon you start to open the throttle).
One other thing I tried was unplugging the MAF to see what happens. When I did this the car did start and idle kinda ok (no poping sounds) - however as soon as I gave it any gas at all, the engine would die. I tried this several times. It is my understanding that the car should be able to drive in limp home mode with the MAF disconnected. Does this tell us anything ?

Thanks in advance for your continued help.

Ok. Plugs and wires double-checked.

Driveable with MAS unplugged is half-fairytale. Idle, sure. Driving? Not so much.

Plugs are fine. You need more than a few minutes of idling about for them to show anything unless something is very wrong or it’s a top-fuel dragster.

I assume all plugs looked more-or-less like the one shown above.

I listened to the video again and I’m still convinced it sounds like intermittent no-spark and on more than one cylinder.

And given MAS and LH rebuild that still seems like the path to pursue.

I ask for the 3rd time: When you did the intake refresh, did you mess with the LH harness grounds? These are the ones bolted to standoffs on the rear of the block.

Did you loosen them? Clean them? Examine them?

If you need a picture I can post one.

Yes - all plugs looked exactly the same

You used the word Grounds (plural). As far as I can see there is only the one ground wire back there and yes it was removed, cleaned and reinstalled. Have I missed a wire ? should there be more than one ? If so a photo would be much appreciated

Giddyupp wrote: ↑Tue Jul 16, 2024 1:39 pm You used the word Grounds (plural). As far as I can see there is only the one ground wire back there and yes it was removed, cleaned and reinstalled. Have I missed a wire ? should there be more than one ? If so a photo would be much appreciated

Pictures below.

But, before that, what means do you have to test/observe spark? Timing light? Oscilloscope? Anything?

There are two ground points. One for EZK and stuff(*), and one for LH and stuff.

(*) I don’t have the wiring diagrams memorized.

Whichever one you messed with, make sure the bolt is not loose.

Whichever one you didn’t mess with, observe its condition. Unless you removed its bolt, the bolt is unlikely to be loose. But, you can check its tightness to be sure. Cleaning it, while maybe a good idea, is unlikely to solve this problem. *if* the ground is the problem it’s because the bolt is loose or the terminal is hanging on by one strand of copper.

The obvious ground:



The not-obvious ground terminal is hidden under the P-clamp for the LH harness. It may be difficult to observe its condition without a small inspection mirror. If you unbolted the cable pulley console during your refresh then definitely make sure it’s where it is supposed to be. The arrow points to the shrink wrap for the terminal:

Man, that looks so clean. Very nice!

I had identical symptoms with a loose LH ground in the past with a long thread on Rennlist related to it.

Given your proximity to it during the intake refresh, you may have loosened parts up back there or disturbed it. Take the air box and MAF out. You can loosen the LH and EZK grounds to remove and clean. Chase the threads on the mounting bolts and lightly sand the surface of the mounting point so it has a REALLY GOOD contact.

Reply after that is done and let us know. Agree with Worf on how to trouble shoot this.

That was it - Wow I just can't believe it. I had loosened the passenger side bolt that holds the main harness to give me some extra access (had no idea there was a ground wire there).

I was checking all around per your advice and was literally about to give up again as I could not see the ground wire at all. It wasn't until I reached in to wiggle the harness that I saw it was extremely loose - I guess I never did it back up again after the top end refresh. Not really sure how it ran ok immediately after that - maybe the bolt has worked looser over time. It was completely loose with almost no thread hanging on. All tightened up and "viola" started right up - idle right around 800rpm (cold) - no poping at all. So happy - Reckon Ive be chasing this for 2-3 months now. Thank you so much for your help with this. won't get a chance to take for a drive for until tomorrow but I pretty confident thats it. Thankyou thank you thank you !

Good first indications of success.

Now given it three or four good heat cycles (e.g. test drives, …) before you declare victory.

And yes, any finger tight (or less) bolt subject to heat cycles and vibration will work itself loose.

Im gonna call this a victory at this point. Been driving all week. Hot starts not a problem and no weird noises at idle. I just took her in for a Smog check and she passed no issues. thanks again for the help

Awesome!