993944S2

I'm also posting this in the racing forum.

Help;
Chasing a miss in the race car. 87 3.2 Carrera. PCA Club Race prepared. Extremely well developed car. Very competent crew. Multiple head scratching trying to diagnose this issue.

Symptoms;
-Slight stumble/miss fire.

When does it happen?
-After 2-3 laps, on track. 12-15 minutes of run time. Does not happen before 10 minute mark.
-Around 4200-6200 RPM, but at WOT.
-Happens in left and right corners. Also happens on straights.
-Seems to appear soon after a shift, or after getting off the throttle.
-Driver reports via radio to crew. Driver describes it as “stumble”.

Thing check, replaced up until now.
Fuel:
-Checked for bad gas.
-Pulled all mechanicals out of fuel cell. Check pick up, lines etc.
-Inspected fuel line to look for crimps or collapse.
-Metered fuel injectors.

Computer & Engine
-Swapped complete engine.
-Swapped ECU.
-Swapped engine wire harness.
-Replaced all plugs
-Metered plug wires
-Connected MSD box. Disconnected MSD box.
-Replaced coil. Twice.
-Steve Wong Chip. Checked twice.

-Replaced ignition switch.
-Replaced air box with rebuilt. Twice.
-Metered all reference sensors.
-Checked TPS.
-Replaced fuel pump.
-Check all wiring/grounds for shorts.
-When engine was swapped, wire loom came with second engine.

Car is equipped with AFM and exhaust temp sensor. When miss happens, exhaust temp pegs. We are running out of things to check/replace. Any suggestions welcome and highly appreciated. If you find the cause beer at the WGI club race is on me…

TDS911

Engine speed sensor or sensor to flywheel gap?

Steve W

a). Replace all the injectors and try again.

b) Dyno the car and see if it's lean up top

Amber Gramps

Yep. Even the slightest crack in the plastic can bleed off the signal. Buy the ones for the 944 and set them super close to the flywheel. Set to far out and you loose count of teeth at higher RPM's. Made a huge difference on my part throttle map.

scarceller

In the distributor check that the springs and weights for doing rotor phasing are working properly and not bound up. Folks don't realize the 3.2 has such a feature to help keep the rotor properly aligned with the posts on the cap at higher RPMs. If the springs break or things bind it could result in cross firing to another post at heavy loads above 4000RPMs.

TT Oversteer

If your EGT pegs concurrently with the miss you must have a lean condition at that moment. Either fuel supply problem or vacuum leak? I recently had a higher rpm miss on my 3.2 that I traced to a swollen and leaky rubber sleeve that joins the two intake manifold castings. Check fuel pressure and vacuum leaks.

whalebird

bench test the AFM for proper metering as well as the intake air temp sensor. I have found several AFMs metered correctly, but were out of spec at the temp sensor. The barometer(altitude sensor) has an easy test too.

993944S2

Thank you everyone for the suggestions. No luck yet, but today we start installing a complete new wire harness. Boy, won't that be fun!! Will update.

rusnak

Do you have a lightened flywheel, or the stock heavy one? The ECM will cut the fuel flow when you lift. You can see the effect when you hook up an AF gauge and apply load.

Droops83

Do you have a wideband O2 sensor setup or know what the air/fuel ratio is when the engine is missing? Knowing this will be critical.

High EGTs can be caused by way too lean OR by way too rich air/fuel ratios, or ignition timing that is seriously off (the earlier suggestion to check the mechanical advance of the distributor is very good), so EGT alone is not enough to go by. Did you try a different chip when you swapped DMEs?

If the stumble/miss were caused by an overly rich condition, you would probably be seeing/smelling back smoke from the exhaust when it occurs and the tailpipes and spark plugs would likely show black soot.

It sounds like this is caused by a lean condition, but this should be checked with a wideband O2 sensor. Have you checked fuel pressure AND delivery volume, both with and without vacuum at the fuel pressure regulator?

Also, check that your fuel tank/cell is vented properly, and that the vent hose isn't kinked or pinched. If there is no vent, the fuel pump will try to collapse the tank/bladder from the suction and will not pump enough fuel volume to the engine (though pressure will likely be within spec).

Good luck and keep us posted.

scarceller

One more thing that can cause this under heavy load is a faulty injector that internally shorts out because of compromised coil windings. Under heavy loads the inj pulse widths are very high (near 8ms) and can heat the coil in the injector which could result in the coil shorting out. Because all 6 injectors are wired in parallel (batch injection) if one shorts out it shorts out all 6 and then fuel flow stops instantly because the DME's Fuel transistor shuts off.

I'd test each injector with a OHM meter to be sure they are all in the 2-3ohm range.

It's rare but I have seen this happen.

993944S2

Update: New (used) wire harness has been installed. We metered everything before installing. We also cleaned up a few other misc. electrical connections. DE on Monday will be used for testing. Thank you very much for all the suggestions. I will update soon. Rennlist is great!

993944S2

Update; New harness installed. Same problem. Two sessions of 15 minutes each. No problem. After lunch, 45 minute session and the problem shows itself. I think we know for sure that it's heat related as the car always runs well for the first 15 minutes of the day, then as it warms the problem reappears.

scarceller

I was going to suggest replacing the fuel injectors. But in your first post you said this also happened with a totally different engine? Did those 2 engines share any common components?

Does the tach stay accurate during the failure? Or does it drop to 0RPM?

theiceman

This may sound really dumb as you have replaced the engine .. but here goes anyway ...

did you jumper out the CHT at the wiring harness after it warms up ?