Engine Misses When Warm

[OK-944]

Still not quite there!

In a previous post I’d mentioned how, during my car’s first, post-rebuild start, with a ten-minute high-idle (1.5-2Krpm’s)…that the engine caught and ran - with good oil pressure in addition to all other functions behaving as they should.

During a second, ten-minute high idle, the engine ran roughly - feeling like a fairly rhythmic miss (like one or two cylinders not getting spark or fuel) for about three minutes before smoothing out.

Fast forward: with oil now changed to a “break in” formula, I started the engine once more and let it warm up for a few minutes prior to what was to be a ring-setting (acceleration/deceleration) run - and the engine ran smoothly until I’d gotten about a tenth of a mile past my driveway…at which point it once again began to behave like one or possibly two cylinders were not igniting, and I was able to turn around and limp back to my garage.

Before I go further…let me add that while I can smell no fuel as the engine is running, it is after I turn it off and let it cool for awhile that I can indeed smell some fuel - but mostly just the fuel lines themselves, which I’d just replaced and covered with insultherm fabric heat wrap (from Lindsey racing). In fact, when the engine is cooled completely, the insultherm fabric, in addition to smelling mildly gassy…feels a bit moist and cool - as if from the evaporating gas.

Here’s where it gets interesting…I can smell no gas from the actual fuel line connections - except for perhaps the faintest whiff from the pressure regulator. Its almost as if the new lines are slightly porous - or perhaps had experienced some huge amount of over-pressure.

I’m pretty sure that during a high fuel pressure condition, there is a feedback loop from the injectors which causes the engine to cut out - but perhaps a particular injector would cut out before the others, which facilitates a “limp home” condition?

More data: Engine sensors are new. DME relay is new. Alternator is tested and putting out nicely. I cleaned the injectors and replaced their filters. Oil pressure, as mentioned, seems good. Fans come on as engine warms. Engine temp indicator seems to work…although does not seem to rise as much as before the fans come on. (hmmm…fan thermostat works independently of engine temp sensor - so maybe the sensor is bad?). I’ve already opened the ECU/DME and looked everything over with a magnifier, and re-flowed one of the coil driver contacts (which looked pretty good anyway). Coil primary resistance, at .3ohms, might be a little low (spec is .4 to .6) - but two new Bosch coils also tested at .3. Secondary resistance is 5.4Kohms - which is a bit lower than the new coils but within spec. (One of the new coils actually tested 7.2Kohms for secondary…which is the max allowable).

In my one experience with a bad coil (1970 Volvo 140 wagon), the engine stumbled erratically. The “stumble” of my current 944 engine is regular and in sync with rpm’s - more like a single plug no-spark or single injector cutout.

I’d gotten very familiar with ICV issues in the past and have ruled this out in this case. While I have not cracked the case on the AFM to see if the needle track is excessively worn…I would not think this would be the cause, as the engine seems to miss with a definite, consistent rhythm. The TPS clicks exactly where it should to indicate idle, and the idle speed is appx. 900rpms. when the engine is running smoothly just after startup.

My thoughts on where to check? To first pull the spark plugs to see if one or more smells gassy - which would indicate a no-spark condition for that or those plug(s). Maybe go ahead and replace the engine temp sensor - which, while it did clean up nicely, is still the original that came with the car.

My gut feeling? That an injector is cutting out as the engine warms…either because of an iffy electrical contact which worsens at it warms up, or because of a temporary over-pressure condition, which could indicate that either a fuel regulator or damper might be going bad.

Sorry to have run on so long with the above…but if anyone could chime in at this point with suggestions - that would be great!

[mytrplseven]

I'd recommend you change the DME temp sensor (blue top in front of engine near water neck. It's an easy fix and sounds like the answer to your problem. Good luck.

[OK-944]

My understanding of the DME temp sensor is that it helps to provide smooth performance while the engine is still cold, while my engine runs smoothly until it becomes warm.

[walfreyydo]

DME temp sensor tells the DME whether the car is warm or cold. If the sensor is bad it could think the engine is cold, when it is in fact warm, thereby overly increasing the fuel mixture (as if it were a cold start). This sensor can be tested (see bottom of this page: Sensors and Gauges - Information, Troubleshooting, and Testing)

I would also put a pressure gauge and see what your fuel pressures are, if they are high I would check the FPR or vacuum leading to it. If its low, check your filter or for any obstruction.

I would also just check spark plugs, verify your gap. Cap, rotor, wires all new? Your issue sounds ignition related if its a rhythmic constant missing, but could also be fuel if its related to a specific injector. TPS is something you could also check - very easy to do.

PS-not smelling gas from the fuel lines is a good thing, you should not smell any fumes from your lines. You can buy spark testers (or a timing light) which will tell you if you dont have spark to a cylinder.