brinckie

Hi All

I'm having a strange issue. I took my 1989 928S4 out to run an errand yesterday. Its been running great and just had the oil changed. After about 20 minutes of driving there was a pronounced lag in the acceleration and the ride got a bit choppy. it almost felt like I was driving over graded road. I managed to limp back to my house. No warning lights, gauges seem fine, fluid levels seem fine. It has about a quarter tank of gas (89 octane). I took it for a quick spin this morning and after about 10 minutes of driving I started feeling the lag again so I returned home.

Any guesses on what's going on here?

Kevin in Atlanta

Assuming this is an A/T, last time you checked crankshaft end play? Could be TBF.

Mrmerlin

As Kevin suggested,
Has the crank end play ever been checked?

check the coil wires see if they are touching any metal on their run, if so then the wire may be damaged
pull them off see if they are corroded.

Open the computer cover by lifting the floor mat,
see if the relay is triggering with a red or green LED lamp illuminated,
if so then one of the ignitions has shut down It
If so then one of the temp sensors could cave corroded connectors
Read the WSM for trouble shooting protocol

Kevin in Atlanta

If you have the old oil filter now is the time to cut it open and look for metal.

If you have not disposed of the oil run it through a paint or coffee filter.

dr bob

Don't forget "relay-relay-relay-relay" after it passes the crankshaft end-play test for TBF. I had some funny similar symptoms as my fuel pump relay was going south (while the car was going north...).

928 DesMoines

I'm not even close to the same league of professionals that have already commented on your situation. But I had a similar issue on a 1990 S4 I used to own. Not quite a dragging feel; more of a "loss of power" feel. Car would start fine, run fine for about 10 minutes of varied time under all conditions, but then all of a sudden, it would just lose power. (not stall or anything like that). I could pull over, turn the car off, sit, start and go another 10 minutes or so...… until I got home. Turned out one of the ignition modules was failing and would cut off 4 of 8 cylinders. I ended up swapping out the ignition modules. Easy job, not costly. I used bosch. condition fixed.
I also changed my fuel filter (but that was not the root issue).

Like the pro's have said here though, check your end play on the flex plate. Also a fairly easy job and should be the very first thing to do either BEFORE buying a 928; or at least be the day 1 task after pulling it into the driveway.

brinckie

Thanks for all the advice. The flex plate was checked by a local 928 pro before I bought the car six months ago. I’ll see if a new relay helps.

Mrmerlin

Inspect coil wires next step,
then watch for the red/green LED on the IM relay when the engine starts to run poorly
NOTE dont keep the engine running any longer than necessary with it not running properly as the driveshaft can be snapped

dr bob

Four new relays. The Fuel Pump, LH, EZK an X-bus relays are candidates. Once you are laying there on the floor with the cover open and the battery disconnected, replacing four is little extra effort after the first one. Just Do It. Use new Genuine Bosch relays from a reliable source. Too many knockoffs littering the market these days.

worf928

This—^ is also a good “internet” guess.

@brinckie Post your general location. You might be close to another 928 owner who might be willing to let you borrow their coil amp assembly.

A couple of notes about the final stage ignition/ coil amps;
- you should not operate the engine for more than a few minutes if the assembly isn’t bolted to the chassis.
- when replacing one or both of the amp modules you need to coat the bottom with CPU paste.

Both of the above ‘cause Heat Transfer Efficiency.

brinckie

Again thanks for the advice. I’m in Raleigh NC. I swapped out the ignition relay last night but didn’t seem to have an effect. I will buy some new ones and give them a try. Is that something that the local car store would have?

brinckie

Where are the coil wires? Is that something I need to get under my car to see?

Shark2626

You have two coils, one on either side of the engine bay mounted on the fenders. The coil wires connect to the coils and then to the distributor caps, the caps receive your spark plug wires on their tops so finding the coil wires on the bottom of the caps should be easy.

Ignition modules rarely fail, save that one for last on your list of guesses.

Merlin told you about your car’s ability to shut down half of the spark plugs under certain conditions, you should reread his posts about that and investigate. When was the last time you did a basic tune up on the car? Caps, rotors, plugs, wires...

Do you have a knowledgable 928 mechanic? It may be well worth the cost to pay him a visit.

FredR

I very much doubt you have a TBF issue but irrespective of your current problem if you have none of the recognised TBF mitigations in place you are playing "Russian Roulette with your motor- checks tell you it has not happened already - they do not tell you "it is about to happen next week".

The ignition system is very reliable but problems can eventually happen. If you were running on 4 cylinders I doubt you would be describing it as a "loss of power" more like "the engine is failing"- loss of power is what happens when an injector or an HT lead fails on one cylinder . The symptoms you describe sound more like a coil going south to me. When this happens invariably it takes a few minutes for the thing to heat up and when it does the output starts to break down and the thing does not want to rev. Cool it off and it returns to normal briefly. This type of problem though rare [the 928's coils are very reliable], is masked by the fact that the engine has two coils so one set of 4 cylinders tries to work as normal - the chances of both coils going at the same time are more or less non existant.

Diagnosing such a fault is going to be tricky unless you have an oscilloscope and appropriate probes to hand. Swapping in a replacement coii "trial and error" style is both easy and cheap enough and you should have a spare coil no matter anyway.

Mrmerlin

guys the most common issues with the dual coil ignitions is a failing coil wire its usually the left side.

What happens is that the coil wire starts touching a piece of metal on its short run or the ends get corroded.
This is usually preceded by motor mounts that sag and eventually move the coil wire so it touches something.
That said if you do find the coil wire touching and see what looks like a white dusty area on the wire,
the chances are good the wire needs to be replaced and yes you can buy new coil wires.

Otherwise water can run down the fender and drip onto the coil and eventually get into the connector ends.

Please follow my instructions and report what you find.
At some point Porsche issued a TSB for shorted coil wires ,
so the mechanics wouldn't be replacing lots of other working items and saving labor time