1989 S4 idles fine but wont rev
 

I have a 1989 928 S4 automatic. The car will start up and idle fine almost every time. When i press the gas pedal the car bogs down and will only rev up if i baby the throttle and finesse it. I just changed the wires thinking maybe it was a misfire. The new wires seemed to help a litte but the car still bogs down. When i put it in reverse or drive the car will not accelerate at all. Im wondering if maybe this is a vacuum issue, maf issue, or injectors? My car also seems to run a little rich. Any ideas?

A few questions to start:
When was the last time that it ran fine, and what have you done to it since then? (Other than spark plug wires.)
You say you think it is running rich. Why do you say that?
Any warning lights or oddities that you notice?
Disconnect the battery for 30 minutes. Make sure it is charged. Reconnect and start.
Your symptoms sound like fuel pressure issue (FPR or Damper), but don't start throwing parts at it yet.
Jason

I recently purchased this car. When i first got it there was a bad fuel line. I replaced that and it would start and idle but not rev. The only part i have put on is the wires because the old ones were cracked badly and i figured the issue was just a misfire. I think it is running rich because there is moisture coming out of the exhaust and there is a rich gas smell from the exhaust. I am not seeing any warning lights.

Was the car running when you purchased it or did the problem start after you worked on it?

If you changed out the ignition leads are you sure you got them all back on in the correct locations?

Your symptomatic description would point me in the direction of the MAF not functioning correctly but if what you did created the problem or made matters worse then you need to eliminate why in the first instance. Perhaps you can offer a bit more clarity on the sequence of events.

So it ran fine before you replaced the fuel line?
Which fuel line, maybe there’s a blockage in the return line so the system is over pressured,

First, what FredR said: Check that the spark plug wires are correct. Check paths from dizzy to plugs, then check again plugs to dizzy. It is easy to think you've followed a line correctly but didn't. And yes, one wrong line can cause symptoms like this.

I'm not sure of your skill level or tools, but since you can actually get it to idle I would first check the fuel pressure at the fuel rail test port. This would eliminate fuel pump, check valve or fuel pressure regulator questions. Attached is the test procedure from the Porsche workshop manual. As mentioned by gazfish, a kinked return line would also cause higher pressures (and those higher pressures may have promoted the repaired fuel line failure.) Visually inspect the return lines, look for kinks in rubber portions and damage to metal lines that run along the right (passenger) side under the car, and near the fuel pump.

Pulling one or more spark plug and inspect. If black and carbon covered you are running rich. It may or may not also be wet from unburned fuel. (One of the headaches with fixing a rich system is that while you are trying to figure it out the plugs are fouling and that is not helping things.)

Next, you do have fuel delivery, so if the pressures are correct then it means that either (1) the injectors are either physically bad or restricted OR (2) that the EFI system (LH) is not "telling" the injectors to squirt like they should OR (3) the ignition system is not properly burning the fuel. Each of these is largely independent of the other, with different tests. Personally, after confirming fuel pressure and delivery I would start with (2) the EFI-LH system, because you do have some fuel delivery through the injectors (1) and some spark (3).

A bad MAF is part of the LH system (reason #2) as mentioned by FredR. First, check the electrical connection at the MAF for any damage. (I had very similar conditions, it would start and idle fine, but as soon as you started giving it gas it would act up.) Turned out to be one pin of the connector at the MAF broke away. I was "that close" to replacing the MAF and found the bad connection. Lots of other causes as well, included bad grounds. If you have read more than half of a thread on this forum you already know to check and clean all of your grounds and have a good fully charged battery. All of the grounds are important, but if I recall the most important to proper LH operation is the engine to body ground strap on the right side and the ground point on the back of the engine.

Again, I wouldn't start throwing parts at it just yet. Your money is better spent on the correct tools to test and measure, and you will use the tools again!
Jason

Another thought: has the car been sitting with the same gas in it for an extended period of time? My 89 had been sitting (for several reasons) for much longer than I would have liked; I experienced similar symptoms which thankfully resolved after draining the tank, refilling with fresh gas and running a bottle of fuel injector cleaner through the system.
Just a thought. Good luck.

Make sure both coil wires are firmly in place. I had a simular issue and it turned out to be a coil wire was knocked loose.

Dylan,

The symptoms sound to me like a out of calibration MAF. I would find a friend that has a running 928 and install your MAF into his car to see if the symptoms follow the MAF.

Doesn't cost anything to make a friend. :)

Is there any way to test my maf on my own? Im afraid there aren't any 928 S4 owners in my area

Pull the connector off the MAF and advise if that modulates the behaviour at all. If it behaves the same then the MAF has gone into limp home mode. Basically this is a default one size fits all mode of operation wherein the fuel is set for a load factor just a bit above idle. In such condition the amount of fuel added is fixed so at idle it is rich and trying to operate the motor at any point above the load set point will lean the fuel out rapidly and the motor will die at anything above the slightest pedal pressure- hence the term "Limp home mode".

Thus if you pull the plug and it runs just the same this will tell you that when the plug is connected the MAF is not working- hence what I wrote in post No2. If indeed this is the case then wither the MAF is goosed or the wiring in the connector is- a very common problem with age. MAF's also degrade with age and under read the air flow but generally speaking that does not cause the degree of degradation you describe.

I have a thread about a failure exactly as it’s described the LH has failed
the thread is about my 1988 Grand Prix White restore project
At any rate take your Computers out and plug them into a good running S4 if the failure follows you know what’s going on

Links are always helpful. At posts 55, 56:
https://rennlist.com/forums/928-foru...project-4.html

Stan...the OP mentioned he does not have easy access to swap out computers.
Jason

I'm still a fan of testing first, throwing parts at it last.
Attached are some tests you can perform on MAF and connections from the Workshop Manual with a multimeter, as well as visually check of MAF wire.
Not a lot there, but the idea is to systematically eliminate issues. (per the recommended methods of Porsche.)
If you do not have the Workshop Manual you can download it here: http://www.ligeti.com/928/

Also attached, is a fuse and relay chart for your car if you don't already have one. Print it out, laminate it...it is one "tool" that you will need often.
I wish I had a good wiring diagram scanned for you, but maybe someone else can post one.
Jason

No doubt, but where is the OP located?

There are 928 owners in every state.

There probably is someone closer, but Dylan, you can send me your MAF and I will put it in one of my cars for testing. PM me and I will send you my address.