SC924S

Howdy Folks,

I’ve got a perplexing issue that I need some help with. A couple of weeks ago, my 1988 924S started lurching when hitting a bump in the road. Not all bumps do it, but particularly big pot holes are pretty reliable. The car will drop from around 2500 RPM cruising speed down to idle and then pop right back up. At first I thought the clutch was slipping, but the engine is revving down instead of up so that wouldn’t make sense.

This issue was present before I did a tune up on the car, and is still present now. I’ve replaced: spark plugs, spark plug cables, cap, rotor, vacuum lines, all during a timing belt job recently. This did not seem to impact the issue. I also pulled and replaced the TPS sensor with a new one, which does not seem to have any impact on the problem. I’m curious if anyone else has any suggestions!

Thanks so much,
-James

V2Rocket

so it drops RPM while in gear? or only in neutral?

a test i can suggest based on an issue i had with my early 944 years ago...find a road with sufficient pot holes to make the car do this problem and do a test run.
then get a 12-14 gauge wire and run it from the battery+ to the ignition coil + terminal (where the black wire goes - be sure to disconnect the factory black wire before doing this).
drive down that road again and see if the issue happens still?

SC924S

Just in gear, it can happen while I’m cruising down the highway or accelerating through the gears.

The battery-to-ignition coil is interesting. Is the intent to see if the coil wire is faulty? I don’t notice any other electrical issues, and can rev through the whole RPM range driving down a nicely paved road.

V2Rocket

i had intermittent issues for a few years where the engine would just cut out randomly while driving.
it wouldn't want to restart, jut crank and crank and crank. i would climb under the dash and wiggle relays and wires and then eventually it would work again.


one time it happened at 4am on an interstate - i had a random piece of wire in the glovebox and had some hunch and connected the coil to the battery.
the car fired up first time and got me home no issue.

turns out the pin/socket that that black wire runs through in the central electrical panel was burned and made spotty connection sometimes when it got jostled.
i redid some wiring under the dash and ran a new wire from the ignition switch all the way to the coil, eliminating the factory black wire and bypassing that pin/socket, and never had the issue again for the next 6 years (when i ripped out all the factory wiring as part of an engine swap).

walfreyydo

Check your throttle cable assembly, otherwise agree with the above regarding loose electrical connections, including the engine grounds (bellhousing grounds on a 944, not sure on the 924) and your TPS connector.

SC924S

Rocket, I’ll give that a look! I appreciate the help. The electrical system seems to be unaffected (radio keeps going), but I’ll check around at the pins as well.

As for the TPS, I’ve heard that I may be able to drive around with the TPS disconnected to see if that helps or changes the issue. Is that true? How does the TPS support the engine management system?

V2Rocket

on a non-turbo the TPS only tells the DME when it should be idling or when it should go to it's wide-open-throttle maps.
there is nothing "in between", all that is determined by the AFM signal. i suppose the AFM door might be getting jostled over bumps but that would only affect it for an instant

SC924S

It really is only for an instant. Driving along, 3000 RPM, hit a bump, needle drops to 1000 and then pops right back up.

AFM barn door wasn’t something I had considered. That would explain the immediacy of the change. I’ll look in to that as well! I may unplug the TPS on my drive home today to see if that removes the issue.

Jfrahm

DME relay maybe. Try testing with the jumper wire in place. They fail due to the internal relays hanging off the circuit board, bad solder joints.
Ignition switch is a good suggestion.

I could also see it being the knock sensor, when they go bad they can get chatty and cause a lot of timing pullout. Disconnect it?

If you are still stumped maybe take a video of the exhaust to see if it goes rich and blows black smoke, that might indicate wiring to the temp sensor or similar.

SC924S

Thanks Jfraham, I just got delivered my F9Tech DME relay so I’ll plug that in later and see if that fixes it. I wasn’t expecting a fuel issue, but it could be!

I don’t think the 924S has a knock sensor, so I’ll count that one out for now.

The exhaust video is an interesting suggestion. Maybe I’ll get someone to drive behind me and see if they see anything!

Jfrahm

The DME relay controls both the fuel pump... and the DME!

> I don’t think the 924S has a knock sensor, so I’ll count that one out for now.

You are probably right, seems weird but the 928 didn't get them until 1987 and the 951's knock sensor was wired to the KLR and not the DME. Some prosaic Audi's had them by 1985.
The 928 gained a fair bit of power from having knock detection, adding it to a 924S/944 would be a nice upgrade. In all these years no one has developed a spark box with knock detection, hmm.

SC924S

Alright folks, a few updates:

I disconnected the Throttle Position Sensor on the car for my drive to work this morning. The car smelled like it was running a bit rich, but either way it did not fix the issue. The lurching was still present, although idle felt a bit smoother.

I plan on reconnecting the TPS for my drive home and swapping out the DME relay for my new one to see if that provides any benefit. I’m hoping to keep only one variable at a time changing to avoid getting more turned around. As of right now, my hunch is the AFM may be getting jostled or even flap closed inadvertently over bumps, but good to rule out a few other items.

Thanks for your suggestions! Keep them coming!