1993 928 GTS hesitation
 

Working on a 93 GTS.

It is cobalt blue, 5 speed, 16,000km (all original).

The car has always had a hesitation issue. By this I mean that the car will act like its hit a rev limiter and then cut in/out. Over a year ago, I did replace some clutch bearings due to slight noise, and the owner told me about the hesitation.
I went in to replace the reference sensor and found that the engine harness had been destroyed by a pervious mechanic. All the shielding was cut, ground wires not where they should be sort of a deal.

I replaced the sensor and recommended a good used harness.

Well our schedules finally aligned and we replaced the engine harness.
The problem was still there.
I have bypassed the IMS and the problem got better but is still there.

We have swapped out with a different MAF,
checked vacuum,
used water for removing possible carbon build up,
swapped in LH and EZK from my car w/ PEMS car has no faults showing up (though I cannot test drive and log due to a crap battery), the car did run for a few minutes flawlessly, but upon a restart the problems returned.
New relays for EZK LH and FP.
Dampers and FPR all have been checked and have no signs of fuel leaking
swapped to known good final stages
cleaned final stages ground,
cleaned engine harness grounds

I now have removed the in tank pump (hose was 100% intact). But of course the knurled insert from the tank decided it wasnt going to stay in :grr: So now we are trying to repair that, and test it with just a strainer as I found that the soldering on the in tank pump was not the best job.

But I am curious as to if anyone else has come across this and what the problem ended up being.

It currently seems to hesitate the most at higher RPMs, it does it both at WOT and cruise.

coils/coil wires/plug wires?

Not sure how bypassing the IMS can make it somewhat better, I thought that was an all or none kind of deal.

If you can't fix it, I will be the high bidder on the car in 2018...

Could it be the ign monitoring system?.. put in a bypass relay and see if it changes?

original plug leads had same symptoms as the magnacores that are in it now. Though admittedly I would like to put in a known good set that I have.
All leads appear good with no corrosion.

Have two new coils and am going to put them in next I think.

Tom,
IMS has been bypassed, so no longer a possibility.

I have finished up the tank repair, appears to be holding, so no in tank pump, problem still persists.

The car does currently have Bosch W7DSC instead of WR7DC+, they look good, and are gapped to around .025-.028" currently.
I'm thinking swapping them out for WR7DC+ may not be a bad idea.
Unfortunately I am working on this car at his house and dont have all my tools here (4-5 hours away.....)

Put in the new coils.
The buried coil negative lead was not properly tight. The lock washer was crushed but it was loose.
I replaced them anyways.

The owner could not get it to reproduce the symptoms, but it could be the cold air or something else.
We are going to insure the car tomorrow morning and will do a proper test of the car to ensure that it has been corrected.

Coils would be my next try, too. Make sure that the coil wires are the "correct style" for the coils that are on the car. There are two different styles. And makes sure the coil wires "push" into the coils far enough...sometimes those black plastic covers keep the coil wires from making contact.

Since the car has done this for a long time...how about a bad/cracked cap under the plastic cover? I've seen this, several times. Check the resistance on the rotor and make sure the spark doesn't have to "jump" a bad resistor. If the caps look good....make sure you pull all the ignition wires out of the caps and check for any "green" connectors. Same thing at the spark plugs.

Hey Greg,

Thanks for chiming in.
Cap and rotor are fairly new. Maybe 3k km. problem was there before and after.

I'm also aware of the coil change in 89 and confirmed that the coil leads are the correct style.

The lead ends were checked for corrosion almost first off. Though I didn't like the feel of the magnacore connections on any of the pieces.
I've already recommended that these leads be changed to the leads made for me as the look much more factory and I've never had a glitch from any I've installed.

Hitting a Rev Limiter would indicate an ignition issue and as the EZK was swapped the coils, wires, disti caps and rotors are suspect.

Spark plugs are cheap and easilly replaced so Id start there.

Coils had no change in the behaviour. :(

After 10-15 min of hard driving the problem came back.
I disconnected the brain and unplugged the O2 sensor to see if it was some form of adaption. No change, problem happened just the same.

Replaced the plugs with Bosch WR7DC+, car ran for a whole 30 sec and stalled out.
Fuel pump is seized, reversed polarity, no change.

So will be replacing pump next......

Intermittent crank position sensor?

Sounds like it could be a weak fuel pump, not delivering enough pressure. I guess you find out when it is changed.

CPS shows good with no fault on tach or STII.
Had same problems before and after sensor changed.

could this be dirty injectors?

jack the car up and check the fuel lines along the bottom of the car, any line thats crushed might be a cause.
Clean the ground on the rear of the V on the engine
clean all of the grounds above the CE panel..
After rereading what you have done,
I would also suggest to put in a new set of ignition wires including 2 new coil wires,
Use the stock Berus
Roger sells them, they are expensive,
but I have not had any issues with a new set,
they usually will last 15 years or more.
Make sure the coil wires dont touch anything along their run.

Hey Stan,

I will be testing new leads next I think.
New pump still has same issues :(

I looked under the car and see no damage under the car at all.