idle stumble/dying on first start from cold

Otter74 · 11-14-2019, 09:09 AM

This is a minor issue, but one that appeared only recently so something changed, but I haven't figured out what.

When my car is cold and I start it, it stumbles a bit and will often die unless I give it a little bit of throttle for a few seconds. This only happens on the first dead-cold start and only lasts for a few seconds. Subsequent starts are fine.

Car is an SC with a Megasquirt running fuel and ignition and an idle air control valve. I did a valve adjustment (valves are looser than before) in June and then fixed a stripped #6 spark plug hole when a plug blew out on a couple days later on a trip to WV. This behavior appeared since then (though i can't remember if it was after the valve adjustment or after I got the car home and fixed it.

I haven't touched anything else on the car.

In principle I suppose there could be a vacuum leak. I'll check the IACV circuit first (Jeep Cherokee IACV). Anything obvious that I'm not thinking of?

911SauCy · 911SauCy's Garage

Outdoor ambient temp change.

What was temp when tuned versus when issue became apparent?

Otter74 · 11-18-2019, 09:06 AM

The PO did the tuning in Atlanta, so not very cold, but the behavior has been evident since summer in temps ranging from ~80F to ~40F.

911SauCy · 911SauCy's Garage

No correlation to cold temps, cold start issues?

Otter74 · 11-21-2019, 12:42 PM

No, it's happened in a fairly wide range of ambient temperatures. Only dead cold engine, and only for a few seconds.

al lkosmal · 11-21-2019, 05:23 PM

I agree that the IAC may be suspect...the fact that you are having to use the pedal to keep it running (adding air by opening the butterfly(s))

However, if the IAC is working fine....I'd run the following test.....

When you initially turn the key to the run position, the M'squirt provides a prime pulse....it may be that you need to increase the prime pulse........and/or the After Start Enrichment.

I'd try this 1st. Tomorrow morning, prior to starting.....switch the ignition from off to run (not into the spring return starter section, just from off to run) three times. This will turn the M'squirt on and off and provide 3 Prime Pulses......several times the typical prime pulse.......if your 911 starts right up and stays running....increase your prime pulse.....

If it is still failing to keep running, increase the ASE (After Start Enrichment) and see if that improves your situation.

Datalog when you do this and you can then associate the stumble/dying with the area that the fueling is in...I.E. cranking. ASE, etc.

regards,
al

gold911 · 11-22-2019, 10:22 AM

This is going to sound crazy, but my sc did the same thing.. for years. It would stumble once on cold start up and die. After the first stumble, it worked great. It turned out to be the main coil wire was arcing to ground. For some reason, after the first stumble, maybe it warms up a little, it does not do it. I finally found it because i had the engine cover open and could hear it "snap" during this short cold start. May or may not be your issue.

stlrj · 11-23-2019, 11:06 AM

Stumble is usually an ignition issue. Concentrate on that. Take a look inside your distributor cap. Look for signs of carbon tracking.

Otter74 · 11-24-2019, 02:45 PM

@stlrj, no distributor cap - car is running on Ford EDIS ignition.

@al, thanks. After I check operation of the IACV circuit I'll do that. However, the car has run without this minor issue for about 17k miles in my possession and maybe another 5k with the PO, and I haven't touched the Megasquirt. Changing settings there will be a last resort.