930 hesitation under load

[gax]

thought i'll ask the collective for some assistance to an issue I am been trying to diagnose, my 79 930 ROW is all stock apart from one of Brians mufflers.

The car has a slight misfire/hesitation under load between 1200/2500 rpm. Now this only happens when the car has been driven for a while and then allowed to sit, possible related so heat soak of some sort. Once restarted then the issue appears, but not always.

If I start and drive the car first thing in the morning and drive it for hours it's perfectly fine, no issues. I have replace the following just to keep the maintenance up which has not helped so far, front and rear fuel pumps, rebuilt WUR, plugs, filters etc.

Any thoughts what I should look at next.

Thanks Greg

[Alan L]

The WUR settings and CIS pressures would help. It may be a leaness. Once you get above 2500 you are close to boost = more fuel.
It could be a slightly off injector, or one cylinder slightly leaner than the others. Quite a time consuming process if nothing pops up early as an issue. May even be a plug/lead breaking down - but more likely at higher revs.
process of elimination really. Throwing parts at it is a blunt way to diagnose it.
Regards
Alan

[356911930]

Another long shot, but, it could be your CDI box. Can you borrow a known good CDI to run for awhile to see if it makes a difference? When these go bad they usually shut down completely until they cool off. Some guys have carried an ice chest in he car with a bag if ice and place the bag of ice on the CDI when the symptoms occur to see if it temporally cures the problem.

best of luck,

Rahl

[gax]

Thanks for the replies, Alan this occurs pretty much off boost, when pulling away in 2nd for example you can feel the car buck slightly on light throttle, goes away as the revs climb.

just went out again for a drive after a 2 hour lunch break and the problem has disappeared, car is running fine. I hate these intermittent issues.

Would the CDI or coil cause such an issue? I thought when the CDI goes bad it just does not fire.

[Alan L]

When the engine is cold the WUR runs richer. Which is why I am suspecting a leaness somewhere - either across the board with fuel settings (not uncommon to have a lean spot on light throttle) or an uneven flow thru the fuel head - one or two cylinders lean.
But based on info to date - hard to eliminate just about anything.
Thats why the fuel info- CIS pressures, and ideally AFRs would help.
Alan

[356911930]

Check the fuel pressures with a CIS guage kit and the "green" wire to the distributor along with the vacuum line to the distributor. I would also check the fuel injectors for correct spray pattern and volume.

And, yes, I believe the CDI can have reduced spark intensity when hot just as it can completely go dead. I am not an expert but worth eliminating as a possible cause. Swapping out with a know good working CDI would be a good strategy if a fellow 930 owner has one handy. I have a spare that I keep just for this type of testing but it is an eight plug for 3.0 930.


Rahl

[T77911S]

check for air leaks

[Alan L]

Yes an air leak would do same - leaness, but once on boost rich. Good point. Spend some time looking for leaks. 90% of the time people are trying to solve an issue that may be a leak, they usually find something. Whether it is the prime cause varies, but any leaks eliminated = better performance. The system calibrates on the basis of No leaks.
Alan

[Alan L]

The other option would be to check flow at FD - for each cylinder. Basically pull injectors and hook up to lines again. Place small collector jars under each one (about 100ml) and run pumps until jars about 3/4 full and measure fuel collected. If any 10% less than the max flow, can bump them up. Can tell you how. Need to remove IC to do this.
Useful to calibrate FD anyway - costs nothing.
My SC had a stumble like that low throttle 1500-2200 when warm. Combination of airleaks at injector O rings and fuel flow at FD. = a couple of lean(er) cylinders.
But first I would go for airleaks because common problem and problem will never be really fixed if you still have leaks. Lots of threads on airleaks.
Regards
Alan