Acting like temporary vapor lock. Help welcome

This issue has been my nemesis. '96 gmc k1500 suburban. 350 eng. 184,000 miles.

Immediate start cold. Engine compartment hot, if it sits shut off for 15 min or more = 4-5 seconds acts like vapor lock, then starts right up. No puff of smoke upon start. No gurgling flooded sound. Once started seems to perform fine.

Obd stored 1code around time this issue started. P1351 - short across or bad ground ignition coil. Installed new bwd coil, code went away. Coil I replaced was original.

Pulled plugs. All nice light brown, each looks identical in color.

Had two repairs done prior to this problem:

Gm fuel pump replaced by gm dealer 1,700 miles prior to this issue.
One cat replaced 1,500 miles prior.

Think bwd coil develops too much resistance when eng compartment hot? Never had problem with bwd quality, but nothing would surprise me anymore. Did I waste money not going with oe?

Crap fuel pump? Dealer insists does not sound like fuel pump issue, but my gut says it could be.

Haven’t tested cat back pressure, but doesn’t seem like a cat issue. Does it?

Maybe a leaky injector so it acts flooded?
Run some Techron through to see what happens.

Correction. P1351 code was for short across or bad ground of icm, not coil. I swapped out both coil and icm at same time. Both bwd new.

Injectors holding right in spec. Two shops have checked them. No leak down.

Could issue be that I did not put the oe Delphi coil and icm back in? Or is that pretty unlikely?

When you first turn on they key, right before starting, the fuel rail is supposed to pressurize. I wonder whether that is happening.
What happens when you turn they key on to the position right before start and off again. Do that a couple if times. Does it then still act like what you describe?

Hey Remco,

I was using my wife’s I-Pad so typing was excruciatingly tedious. Back on the laptop now.

When I turn the key on, I can hear the fuel pump cycle on for its scheduled 10-second charge. I never start the Burb until after the fuel pump has finished that initial charge shot.

I’ll try doing it a couple times and see if I get a different result. It’s too late for me to do it tonight, but I’ll be able to try it tomorrow and let you know if something changed.

Are you still using ethanol free gas? (
I’d try running normal gas and see what happens.

Hey Remco,

I’ve cycled through quite a few tanks of each since I really started paying attention and there hasn’t been a bit of difference. I was hoping it was something to do with one of the fuels, but nothing changed as I alternated tanks (more than just a few).


Forgot to mention. I did what you mentioned - trying to charge the rails a couple times before starting to see if it made any difference. Once the pump initially charged, I’d pull the key out, let it sit for about 10 seconds, then turn the key back on again. No initial pump charge on the second time around. Maybe because it was just a span of 10 seconds. I have no idea. No difference in behavior.

This is a general statement. I spoke with one of the brightest mechanics I know in this area, just to see if he’s been having any issues with BWD’s ignition components. He said he’s had more electronics parts failures from BWD than he’s been accustomed to over the years. Said he thinks it’s certainly possible that something about the BWD coil and/or ICM is not cutting it for my Burb. Of course he couldn’t say for certain. Just thought I’d spit that out there.

See if the problem happens when you spray some starter fluid into the intake when the engine is hot.

The starter fluid is easy and quick enough. It would at least rule in/out a fuel issue.

I’ve taken back to getting more exercise, which is clearing my mind more these days. While out on my bike today, I re-assessed the symptoms. The most logical explanation I can come up with is that it is electrical resistance in the ignition system components, resistance that goes way up as the entire engine compartment gets brought up to full operating temp.

It could be a fuel pump issue also, perhaps a relay or pressure switch/regulator that, upon running for a long enough time to fully heat the engine compartment, that portion of the fuel delivery system develops high resistance and hence doesn’t function properly. It doesn’t stop functioning, but takes more oomph to do its job.

I swapped out the engine coolant temp sensor with an OE Delphi, but no change. No big deal.

Just a quick thought before I turn in for the night.

Yes indeed; using the starter fluid trick will tell you a lot about the problem real quick. It is a great way to find out which area the trouble is in. I also agree with your logical thoughts about the causes of the trouble.

I forgot about the starting fluid test. I’ll grab a can of it when I’m at the parts house next and post what happened after I try it.

Just spitting this out before I shut the computer off.

Anyone think this could be due to running synthetic oil in it? At least this year of GM has a fuel pump cutoff inside the oil pressure sensor/sender unit. Fuel pump is prohibited from pumping during cranking until oil pressure has passed a certain psi threshhold.

This is a '96, back when synthetic was the exception, not the standard. Since synthetic base is a thinner (arguably much thinner) composition, I’m wondering if it’s possible that OE specs for the oil pressure sensor/sender, since they were obviously set for the thicker base of conventional motor oil, simply aren’t appropriate for the thin base of synthetic oils, which offer less resistance to flow (ie. lower oil pressure at start-up).

And maybe with a newer car, you don’t notice it because the oil pump is newer. But my oil pump is almost 17 years old. Since it’s older and it’s pumping the thinner synthetic at re-start, hence lower oil pressure/resistance to flow, the sensor basically tells it to wait a second before energizing the fuel pump at crank.

Just a thought.

If anyone has a thought about this, feel free to chime in.

I am copying this response from a different thread of mine in case the poster wanted to offer any thoughts on this issue.

Hey BustedKnuckles. I won’t run on with this one here because I already have a thread opened for the issue. The cold start issue in a nutshell is motor fires immediately when cold, cranks for 3-5 seconds before firing up and running fine when engine compartment hot. As this issue has continued, I have started seeing a slight bit of black smoke at hot start-up now, suggesting the issue is caused by too rich of a mixture only at hot start-up. I’ll copy this response over to my thread entitled “Acts like vapor lock when hot” (or that’s close to the title) if you want to throw in any opinions on it.

Oil pressure sender/sensor/fuel pump cutoff is one of only components I replaced with other than OE. Just thinking out loud here…if the sensor reads that oil pressure is just a touch too low (perhaps because of the synthetic I’m using that wasn’t figured into the design of the 96 motors) and kills fuel pump for 3-5 seconds while oil pressure builds during cranking, this couldn’t account for mixture being too rich at hot start, could it?

Put another way…if fuel pump is cut off by sensor for 3-5 seconds, could the motor generate enough vacuum during cranking to still suck fuel from the rails into the cylinders so that after the 3-5 second delay, pump kicks on and recharges rails, motor calls for more fuel into cylinders, and I can see a slight puff of black smoke out the tail pipe upon startup?