renopor

hi all, i have an 89 S4 928, drive it everyday. now i better start at the begining, i own her about a year and a half when i got her, it drove ok, but not great. it was my first 928 so didnt really know what power was like in them, but i knew it was down a bit.
it also has a major mis or hesitation which starts at 4000 rpms and continues every couple of hundred revs all the way to the top, it only really happens when you are driving it hard, if you build the revs slowly it wont hesitate. its like hitting a rev limiter that kicks in and out every few rev as they build, like 4000, 4300, 4500 and so on.
i put new rotors and dis caps on, new plugs and filters and got a little power gain.
i bought a dura matic system to plug her in a couple of months later and it showed up that the knock sensor were playing up, so i done the job, took off inlet maniflod etc, and check all vac lines etc. everything seems fine. put it all back together and again a little gain in power.

bought a new fuel pump and while i was down there i found i had a second pump in the tank, when i checked in on the bench it didnt move, so i thought i had sorted it! anyway, fixed the little pump and put it all back together and again a little gain and a little smoother running, but still had the hesitation.

i have noticed that in the morning or when the car is cold it runs much better, you can even hear it in the exhaust, nice and raspy, but the minate it heats up, its down on power again.

now tonight i gave her a serious hard drive, long run on motorway, and hard back road drive, about an hour in all and on the way move, at about 60 mph, i put the foot down, it hit 4000 rpm and hesitated, then missed badly and then bang, a big puf of smoke, i said to myself, here we go, ill be taken the engine out now!, but no, it stop missing and i put the boot down and it has never driven as well, power on tap at low rev and no hesitation high up the rev, its pulling like a train!

any ideas whats happened? now this is all over abot 10 months and only tonight has this puf of smoke happened. i was going to post about it running better in the mornings etc to see if anyone had any ideas whats going on, but now im just hoping it will be running the same when i take her out tomorrow. any ideas lads? cheers thomas

Dean_Fuller

I would of said old cracked plug wires...but maybe not now. I don't have a clue why it would clear up like that.

Mine had a bad hesitation under load but it was the wires. After I changed them all was well.

Hilton

+1 - given it fixed itself when you took the foot of the throttle, rather than needing to turn off the ignition, its not the ignition-monitoring relay.

First things I'd check are cleaning the connectors on the leads to the coils, especially the right-side one, and also the connectors on the ignition amplifiers (under the plastic cover front-left of the car with the hood open).

Then I'd check the plug wires for sparks that indicate the insulating sheathing is cracked from age - run the engine in the dark and look for small sparks around the wires. If you see any, replace the wires.

LASTurbo

I agree, mine did that awhile ago and with new wires,caps,rotors---was good as new!! Problem is....it is not cheap for parts but what is for our beauties!!

mickster

What I write is based on nothing but theory and complete and utter lack of experience but...

Sounds to me like some type of back-pressure in the exhaust system after it got warmed up-maybe there was something gunked up or corroded that was blocking the exhaust somewhere in the path: a pipe, cat or muffler that expanded as it got warm....

What color was the smoke?

Michael

Ducman82

bad fuel?

renopor

little update, went out in her just there and its back to its old tricks. this has been happening for a long time to this car, i check the service history last night and it writen on some of the invoices to check for a miss high up in rev range, but it never said that they sorted it. i have not replaced the leads yet as they look fine, checked for spark leakage etc, ill check all those connections mentioned above. could it be MAF?

Dan87951

If spark plug wires are original get them replaced. Have you replaced the hall sensor? I ask because if the power is down it could be the hall sensor. The hall sensor will retard timing if its not working properly.

mickster

+1

Time to run down some things:

Is the timing right on the car?

What color was the smoke? Blue, White or Black? Blue=Oil dump, White=Antifreeze dump, Black=fuel dump.

Per Ducman82, have you drained the fuel tank ever? When you changed out the fuel pump did you change the filters? Did you change the fuel lines?

You said you checked the vacuum lines-did you test them or just a visual?

Fuel injectors ever cleaned or replaced?

Bad 02 sensor? Bad MAF?

What did the Durametric Tool tell you re: timing and mix and such?

Michael

heinrich

could be anything. don;t drive it.

renopor

its been like this for ages, i drove her today and if someone that never drove a 928 got in they would never know. i have drain the tank and changed the filter which was about six months ago. i tested all of the vac line and there all fine, only had to replace one. i replaced the hall sensor, pig of a job! cthe duramatics wont show any real time data like timing etc, just fault codes and reset them etc. there is no o2 sensor on the car,euro spec. the smoke was black, defo fuel etc. if i check the o2 and its out would it be the maf? also, the first exhaust box has been replaced with just to pipes, so there is no cross over pipe etc, would this coz a problem, the rest of the exhaust is all standard. cheers lads.

renopor

her she is

Dean_Fuller

Try cleaning the 14 pin connector at the right side coil. Disconnect bat first. Does it have the cover??? Washed it lately? Water inside does crazy things.

BTW...both beautiful cars! Right hand drive?

the flyin' scotsman

An ignition system under high load may break down but wouldn't cure itself.

Does the car have the original cat exhaust system?

renopor

i cleaned the connector when i did the hall sensor, havnt washed engine at all.scotsman, thats may thought, iif it where leads they would be gone all the time. was out in her agian today and the heitation still still there but not as bad as on some days. it pull with no hesitation all the way to 5000 then when it stifted up to the next gear it started the hesitation at about 4300 and did it a couple of time before hitting 5 again. it had no cats from the factory, non cat model euro spec. it comes up with a fault on the duramatics system, saying there is a faults with the knock sensor control unit. does anyne know where that is? or is it built into the brain. after reading a few post on here, im thinking it mite be the MAF, but im not sure. it runs fine, no cutting out,or lumpy idle.it runs so much better though when its cold. could that mean its running lean when hot?