98 D1, running a little rough at idle lately but other than that runs pretty good, plugged in the UG today and noticed the ST fuel trim was varying between -20 and -25, and would max out at -25 at idle. LT is 12.5 and numbers track for both banks. Trying to compensate for running rich ? Normally it varies between -10 to +10 and bounces around 0 at idle. Plugs wires coolant temp sensor seem ok, O2’s seem to be swinging normally, haven't dug into it much yet. No codes. Any thoughts, advice, and direction would be most appreciated, thanks !

 

IACV, check it's bore/passages/pintle for carbon. Carburetor cleaner does the trick. Hit the throttle plate and bore too with cleaner, the plate builds up gunk around it's edge from engine oil vapor. Even with a hot engine you'll want some richness as it helps with cylinder cooling. IACV might be getting lazy too depending on how old it is. The LTFT is normal with an overall lean condition so it has to be the idle circuit.

 

Thanks, will do that. I cleaned it up pretty good last summer but who knows, and have been having idle problems for a while now especially with the warmer temperatures. Also, to see what the effect would be, I pulled the connector off the MAF and the whole plate came off and sensor fell apart in my hand…

 

They don't call Lucas the Prince of Darkness for nothing....... Paul Grant has em, way better than aftermarket. I broke mine cleaning it and PM'd Paul, looked brand new and worked better than my original. You might want to ask him if he has an IACV too.

 

New used MAF. 👍👍