Josh B

Ok - with the 3 liter I have a stutter only under load that starts at between 4500 and 5000 RPM. When I first got the car about 6 months ago it was only every once in a while and seemed more pronounced under higher boost settings (like 17 PSI). I've been working with John with Vitesse to get the car tuned up. We were on the dyno the other day and the operator attached a wire to the power lead to the coil pack and the car was doing it every run. We pulled the wire and it stopped doing it - did multiple pulls even one at 17 PSI and no sign of the stutter.

I filled up the car with e85 right after that and it would repeat every time I was under load and reached that RPM - even with the boost turned down. Feels like I am losing spark or dropping a cylinder. Fuel pressure remains constant and A/F starts to climb according to the data logs on the Zietronix. I immedietly lift when I feel the car start doing it.

I started swapping out parts with known good ones to see if I could isolate a failed component. Here is what I have tried swapping out with no luck. Now the car is doing is at 10 psi on pump fuel every time I reach that RPM range under load.

DME
Coil Pack
Wasted Spark control unit
Wiring from wasted spark to coil packs (including power lead which was temp wired direct to the battery)
Speed and ref sensors
Spark plugs (closed up the gap too)
Ignition wires
KLR
SMT6

Anyone have any ideas? Seems like it has to be electrical interference of some type.

Tom M'Guinn

My first thought was ignition, but looks like you swapped just about everything. Is the climbing AFR telling you an injector is not firing right? Any chance an injector is bad, or the harness, or one of the injector resistors is heating up and failing?

refresh951

Josh,

Does it happen only at WOT or does it also occur at PT (TPS< 60%)?

carlege

Do you have the Ice Shark battery cables on the car?

fast951

Now it came back at 10psi? This is a weird one, we tested/swapped about every component, data logged, and nothing changed... (add to your list, different DME/KLR/sensors/software)... I don't think we changed injectors, do you recall?


If it's electrical interference, why would boost affect it? And why it didn't show up on the dyno?

As I mentioned it, I have seen similar (but not exact) behavior on a race car with a broken inner valve spring... It hits certain RPM and goes flat....

If it wasn't so hot, I'll volunteer to keep digging deeper into it...

Josh B

The harness is in like new condition but I suppose there could be something screwy with it. I need to look - might not even have injector resistors on this car...

I don't know that I've testet at 60% but certainly at about 30% and it didn't do it.

No - original battery cables. I think that the ground cable is compromised in spot as I recall - have another set I should probably stick on...

No - didn't change injectors - maybe I should start looking for someone with a spare set I could try out.

It is a stumper for sure.

refresh951

Might try swapping the TPS.

Quinlan

Not to insult you, but I had a similar problem at the recent Watkins Glen Club Races, and it turned out to be a loose hose clamp on the intercooler pipe ... I (and the 5 other 944 guys who were scratching their heads over it) rolled my eyes back when another guy noticed it.

carlege

hell, everything else on the car is upgraded might as well get a new set of battery cables. John woods is working on putting sets together with better cables that should be plug and play He is taking orders on Reutterwerk. if interested

Let me know if you need any help with that.

Josh B

Just seems like the car doing it when the dyno operator hooked on the RPM meter - that has to be a clue... Did it everytime while it was attached and stopped doing it right after the wire was removed.

JohnKoaWood

Sounds like a ghost in the machine..

almost sounds like some sort of induction loop interfering with the coil pack (made worse when the pickup wire was connected on the dyno).

Do you or John have any way of instrumenting each plug wire with a pickup to watch for single cylinder drop out while running? (did the dyno operator have smothing turned on? can he pull the data file up, turn off smothing, and look for odd results from the RPM pickup?)

Similar instrumentation for injectors (since using the DME you are still running batch fire) to watch for a dropped injector.

AFR would climb if NO combustion had occured regardless if it was caused by lack of fuel or lack of ignition. (yes there is fuel in the exhaust, but the o2 isnt being consumed, until it is in the exhaust stream, and then much less completely than if it were in the combustion chamber)

Whats the engine harness look like? Is it original, partially rebuilt using LR style patch cables, or fully rebuilt new?

I have on hand enough to build a patch cable to instrument the injector harness to watch for pulses at each injector (however a scope may be needed to see individual missing pulses at a single injector at 4K, as at 4K the injectors are pulsed with a duration approaching 10ms) much easier to instrument the plug wires with an inductive pickup, connected to a scope, and watch for dropped ignition pulses!

Are you running resistor plugs and or wires?

I also imagine you have double and tripple checked your grounds? How about the DME ground path?

ALSO do you have individual runner EGT probes? MIGHT be able to catch the drop out that way to indicate which cylinder is missing, if in fact one is missing! Other wise you could put it on the dyno, induce the stutter, and IR probe each runner, looking for one varied from the others...

Once upon a time, I purchased a car that under certain conditions would reset loop the computer, and the reset would happen before #4 would recieve its ignition pulse.. $12 in parts later, I fixed the underlying cause, and drove the car for 3 years with no issues...

JohnKoaWood

BAH..had a local car stuttering at 4K in 3rd, 5K in 4th.... retarded the ignition to eliminate cylinder knock, stutter went away!

read the plugs looking for knock, and / or connect a knock counter to the test port (KLR pin 15) to see if the stutter is being caused by knock detection!

Tom M'Guinn

You seem to be saying the reverse of your first post, no? Was it bogging when the wire was on, or off, the coil? Where did he attach it and was it supplying power or just a low impedance signal probe? Where is your WS box located, and how much electronics are you feeding straight off the dme power and ground wires? Have you tried reverting the cap and rotor just to rule out any funky interference/power issues with the WS? Swapping the injectors and checking any injector resistors probably worthwhile too. Is the pin on the flywheel installed as per factory?

fast951

On the dyno, the inductive clamp to pick up RPM was connected to the incorrect wire on the WS harness by the coil-pack causing all sort of interference/noise issues.

No knock, we did many dyno pulls, even one runaway high boost (it hit 22psi) and problem was not there.

Tried different DME, KLR, PB, WS box, WS harness directly off my own DME, Coil pack,...
Speed/Ref don't seem to be the cause as the WS box is in sync and flashing properly (of course, it will not hurt to put the scope on it, but what does it have to do with boost?).. Nothing obvious in the DME based on Motronic Monitor Data Log...

Using a scope will be nice, however, hooking up the scope and running wires while driving is not a safe idea.. So any test must be done on the dyno, but of course, the last dyno session there was no problem...

At least now it's consistent and not moving around all over the RPM and boost range. Which makes it a bit easier to diagnose..

Duke

If you brake boost in that area (4500-5000) to keep the rpm's from climbing, is it constantly missing/stuttering or is it just an initial miss/stutter?

An injector problem would be odd since it only happens at a certain rpm. Even if the injector would fail at a specific pulse width the rpm's would move around at different boost levels.