Flashing Check Engine Light during car wash??

[BeenChevy]

I've got an 01 Yukon that's had the same issue (P0300) when going through the wash. Not so much the undercarriage spray but going through the drying at the end. The SES will usually flash and then go out or flash and remain on for a 5-6 start/drive cycles. Never thought much of it but would be curious to find the exact cause.

Imagine something getting wet or a temporary manifold leak. When the temp is below 30F it will usually stumble a bit until it warms up but usually never kill. Plugging in the block heater when it's that cold or below usually resolves this as well.

I'll try the spray bottle on the spark plug idea. After the plugs and wires were replaced is when I first noticed the issue. My old mechanic seemed to have a lot of trouble with spark plug 5.. didn't get a straight answer out of him as to whether he stripped it out or messed up the intake manifold or what but that was a couple years ago now and the truck runs drive great otherwise.

Any other ideas from the described? Stock intake manifold + air box is in good shape (no cracks). I've got another from an 05 Tahoe I could try if someone really thinks that's a problem, but I think i'd have other codes that went along with a cracked intake manifold such as a MAF or MAP type triggers.

 

[David Paul]

Slight stumbling when cold is often caused by a bad intake manifold gasket. Common problem.

 

[BeenChevy]

Agreed. Still gonna double check the coils and basics/not so basics to narrow it down and be certain.

Other than a smoker (which I don't have). What's a good way to check for gasket leaks. Is brake cleaner the best to use? Or is there anything else that works well and not as corrosive?

 

[SLCHOE]

It's your MAF getting wet somehow in that particular car wash. If you don't know how a MAF works or what it does this is the short course.

A MAF is used to judge how much air is ingested by the engine so the ECM can calculate how much fuel is needed to keep the air/fuel ratio just about 14.7:1 at all times.

The MAF is basically a hot wire that the ECM sends current through, to keep that wire at the same temperature. As more air flows across the wire, the ECM needs to increase current through it to keep it at the same temperature and vice-versa. The ECM knows how much current is needed at a certain velocity of airflow at a few different set-points (idle -no load being one and a few higher RPM benchmarks. Using those figures it can thus interpolate and deduce the differences in current increases/decreases to keep that wire at the same temperature as more or less air coming into the engine (using mathematics and other sensors like the IAT and if equipped a MAP sensor can get this even more accurate. You can see how instantly cooling this hot wire would trigger the ECM into thinking that it has way more air all of the sudden and adjust the fuel trims to richen the mixture by increasing the injector "on-time" (PWM). Of course there really isn't that much airflow so the mixture becomes super-rich and causes misfires (P0300). This flashes the MIL because id this condition was valid, prolonged running under these conditions WOULD CAUSE premature Catalytic Converter FAILURE. Of course it's not valid and after the wire warms back up, the conditions and fuel trims go back to normal and the light goes back off.

OK, so not such a short course I guess. LOL

 

[BeenChevy]

Appreciate the response. Your crash course is good but not air tight. If water was shorting the MAF wouldn't it trigger P0101 and the MAP sensor take over?

 

[SLCHOE]

Appreciate the response. Your crash course is good but not air tight. If water was shorting the MAF wouldn't it trigger P0101 and the MAP sensor take over?

Never said it was "shorting out" the MAF. What water does to a MAF is cool it off, falsely replicating a lot of air passing though. The ECM corrects by adding the needed fuel to keep stoich. Since the air coming in is really at a normal flow for the RPM, the added fuel correction flloods the engine with uneccessary fuel, causing multiple misfires and on both banks.

A shorted MAF would also throw a MAF circuit code.