tylenol

Thanks for looking...I have a 87 s4 5 speed with 52,000 miles. I have owned it about a year and it has never ran right!! (posted about this a couple times but have updates) Completly quit running on day I bought it. Going 70 mph and just quit. Replaced the lh had it shipped home (kansas city) and ran fine for a couple weeks. Started missing out after warmed up. Would fall on its face and then recover and run fine. finally stalled and quit would not restart. Was told by dealer the maf was bad had it rebuilt pulled the fuel tank drained it and filled with fresh gas. Ran fine again a couple weeks. Started missing again but never just quit took it to a local porsche guy who has experience with 928's. Checked all inputs outputs checked all grounds replaced engine tempature switch and coil wire. Still missing so he gave up!!! Drove home today going about 60 starts backfiring/missing will hold enough power to maintain 55 or so and then for no reason will come back to life and pull hard??

The only constant thing that happens is when you start from cold it will idle perfect for about 3 to 5 minutes and then idle will start to bounce around 5 to 900 rpm. When the rpms start to dance around sitting in the garage give it throttle sometimes it will miss sometimes not???

Thanks in advance for any help

Jadz928

The usuals... check out this document on maintanence trends LINK

MAF - you can send it out to be tested, or swap with a know good one.

Check idle switch function per WSM troubleshooting LINK

Check CPS connectors and wires for frays.

cpayne

I put a good known MAF in for previous similar trouble cured the backfiring and poor performance. Had mine rebuilt no problems.
Recently it dropped to 4 cylinders and found the problem to be coil wires.
Pull it into are dark place and start it. Check all the wires both plugs and coil for blue strobes . Clean your grounds too. start with the one under the 14 pin connector as its for the ignition.

oups59

Check the injector wiring harness.

Check also your ignition coils

AO

Exactly how mine acted when the gounds to the wiring harness got loose. Does your tach start going crazy? Mine did.

At the back of the intake, with the airbox removed, if you follow the harness from the firewall you will see where it bolts/grounds to each head on either sire of the MAF. If either or both of these grounds are loose - your car will not be happy.

Hope this helps.

Landseer

^^ mine too.

tylenol

thanks for the tips...i will go to work.

No when it cuts out tach just sits still. I actually went about a mile and a half with it just spittin and sputterin and the tach sat at 2700 or so rpm??

Landseer

Jim had some funny stuff tach happen with a frayed wire, I think in the CPS boot.

I forgot the details at somepoint during the LONG WALK HOME that day.

AO

Well then I don't think it's a loose harness - although it's worth checking.

Here are other potential culprits:
coil/coil wire
distributor cap/rotor
final stage ignition module
fuel pump
MAF/MAF harness
LH

You might also want to click the link in my signature for LH/EZK diagnostics.

Landseer

There's a ground for the ignition amps hiding on the fender just underneath the 14 pin connector (two brown wires with a ring terminal end) that sometimes get forgotten.

Dan87951

Are the ignition wires original?

SteveG

1. I assume you have tried disconnecting the MAF ??

2. It wouldn't seem like the CPS if it registers anything as in "sits still" but sitting still is not normal. I think I would check or replace CPS.

John Speake

If it runs fine until it warms up it could be an O2 sensor problem, as until it gets to temp it runs open loop. This also suggests the MAF is good.

I assume FP, LH and EZK relays are good ?

tylenol

Have not checked relays yet...swapped the maf out with known good one nothing changed.

CPS??? sorry novice here.

Thanks

Mike Frye

Crank position sensor. It tells the system that the car is cranking. Found on top of the bellhousing under the air filter.