Coated exhaust manifold "phenomena"

jayroc · Post by **jayroc** » Sat Jan 14, 2006 10:58 pm

I got a ceramic coated exhaust manifold installed and it lowered my MAT temps a pretty good amount, however I have noticed something strange...
At idle, I noted that the MAT was around 60 with a CLT of about 175-180. So, I shut the car off and come back maybe 30 mins later. MAT is 90-95 and CLT is 130 or so.. Any ideas as to why MAT would increase like that? I thought my sensors might be lying to me, but the best I can figure is that air from inside the engine is creeping back up through the manifold and through the TB butterflys back into where my MAT is mounted. Sound plausible?

PSIG · Post by **PSIG** » Sun Jan 15, 2006 12:43 am

jayroc - this is a common situation referenced as heatsoak. Your engine and sensors are sitting in an underhood 'oven' after shutdown and the heat transfers to the MAT sensor. If you are using a true MAT (IAT) sensor, the temp on startup should correct very rapidly. Those that use a CLT sensor to do the job often have hot-start problems as it can take nearly a minute for some CLT sensors to respond to the cool air entering the engine.

Unless you're having hot start problems there is no need for concern.

Hope that helps,
David

motown menace · Post by **motown menace** » Sun Jan 15, 2006 7:17 am

Did you ever take the same readings before you installed the coated exhaust? Are you saying you didn't get heatsoak before?

jayroc · Post by **jayroc** » Sun Jan 15, 2006 10:44 am

Actually, I was having hot start issues before installing this manifold, I've recently got them sorted out. Mostly pulsewidth and ASE related..

No, I never noticed it before.. At idle the MATs would get as high as 120 and cool after shutdown. Both the CLT and MAT sensors are GM. My cranking pulsewidths are based on CLT only, but I'm not sure how much of a factor MAT is when MS calculates cranking PWs..
I'm not really having a problem, just wanted to know what that was. Heatsoak, eh? I heard of it before, just never thought enough about it to look it up.

PSIG · Post by **PSIG** » Mon Jan 16, 2006 12:01 pm

It seems the hot MATs/IATs cause an afterstart lean condition rather than a cranking PW issue. The engine starts then sags, stumbles or stalls until the IAT stabilizes to the proper (actual ambient) output. Try pulling your IAT and cooling or warming it to ambient to see if it affects your starting. If not - you're good to go.