1998 PCM delay?

[GregRexAdams]

On the two '98 F-bodies that I've tuned there appears to be a delay that isn't in other, later cars. I clocked it on the first car at 3 minutes after startup, the car went from pegging my wideband lean to posting exactly what it is supposed to. I can't find any table or setting for it though. Has anyone else seen this? Is there a fix? This is any startup, not cold starts. Commanded stays the same.

[Frost]

post the tune

[batchfire]

my 99 7.4 vortec does this too

[GregRexAdams]

base airflow settings? cat / air pump / egr ? post a log and tune

First car is a 98 Z/28 with cam and supporting bolt-ons. I don't have a log, but I have the finished tune. It had a tune done before the cam and before I tuned it.

The second car was only a bolt on car, bone stock besides catback exhaust and maybe EGR. I deleted some of his files, so I only posted the stock file. I didn't mess with base airflow on it in tuning. No headers or anything.

[GregRexAdams]

Any other thoughts on this one guys?

[Frost]

Yeah, no logs

Stock 98 tunes (cars) are not off the wideband lean at startup at all unless they or your wideband have a physical problem.

Post a log with wideband and NB votages, MAF freq, MAP, ECT, IAT, and RPM.

This is not a normal thing nor a '98 thing', and there is no 'delay' to normal fueling in the sense that you are speaking of.

File one, from 140-190deg has an OLFA of all 1.0 which will make a lean spot... even with a calibrated MAF/VE, the cars seem to run a little leaner than commanded in OLFA as they warm up.

[GregRexAdams]

I just got home from working on it again. I found the culprit. Amazingly, I found it because someone else didn't do something real well.

While tinkering with the car, we noticed it was at 108* IAT. Now, it is about 36* outside. He starts driving and the car stays lean. When we hit the road were he can really get off idle, the temps drop rapidly, like 60* in a few blocks, at which point the car starts running a better AFR. So, we pull over, and let it idle. It's idling at about 13.8 or so at a 46* IAT (see frame 4665 and go from there). As the IAT climbs, the car leans out and in the 60s it is lean again, and never stopped leaning out. My wideband AFX maxed out in the 90s I think. All the while commanded stayed 14.63.

We drove again, and as it cooled off, it started richening up. This time we popped the hood and looked, and someone had cut the brace right under the airbox out...so that explains the quickly rising IAT.

This compounded the situation...when we would stop to flash the ECM, the temps quickly rose, and the car quickly leaned out, so that by the time I was done tweaking the tune and writing it, the car was hot and thus lean again, appearing to be a startup issue. I believe this was probably a lesser but similar problem on the other '98. Thing was, hood had to be closed, at one point we had the hood open and it wouldn't get over the 60s lol. Too cold.

I couldn't believe the bias was off that much.

Consider this one solved. Guess it's time for a PCM swap.

Edit-went ahead and posted the log tonight.

[LSxpwrdZ]

Interesting because my TA is a 98 and I've cut the bottom of my air lid base as well but I don't have these issues. Maybe because mine is a TA and has a Ram Air hood with no baffles so it acts as if the hood stays open?? After seeing the free mod of cutting the air lid base and it's affects on the IAT, I really don't like that mod. I'm going to actually try to eliminate the air lid all together on my car and make a custom 4" intake to worm down in behind the front bumper cover for my Camaro. On the TA I'm gonna use an LT1 radiator support and make my own FIPK style intake.

[346ci]

I have a 98 with the bottom of the air filter tray cut out and have not had this problem even in cold weather. I guess every car is different

[GregRexAdams]

346ci, I would guess that something is somehow different in your setup, but I couldn't hazard a guess as to what, considering that hot air rising is going to have the same affect on both cars...something on yours is keeping it from getting to the sensor (and therefore hopefully the airbox). Given how fast the temperature dropped, I wouldn't say this is a problem when the car is moving or under normal circumstances. In a GTO it would be due to the slow rates of our IAT sensors. His responded very fast.

LSxpwrdZ, yeah, having an open source of fresh air like the scoops or having an open hood would make a difference. In the summer, I bet it would have been hard to notice even on this car.

The bottom being cut IS NOT the problem, but it allowed the problem to be highlighted. The problem is how the car accounts for that change in temperature...the sensor was obviously responding, but the computer wasn't adjusting as it should have been.

I also believe his intake played a roll however, being it was aluminum (BBK SSI). I think it threw the bias off even more, although I don't really want to get into a "heat soak" debate on intake manifolds.

[LSxpwrdZ]

Yeah I see where you are coming from. I'm in the process of converting my lid style intake over to a custom FIPK style setup using an LT1 radiator support and I'm going to build a box around it that seals to the hood so no heat can get into the air filter. Heat soak is terible and would be alot better dealt with if the IAT sensor would actually respond to temperature change quicker.

I have noticed when sitting still the IAT sensor does rise quite quickly but as soon as I'm moving it drops instantly but does so very slowly. I know for a fact that as soon as I'm moving the air temps should be nearly ambient in the air lid but the heat soak of the sensor itself and the response of the sensor is really slow to show this.

I've also thought about trying out an Air Bridge from a Corvette to locate the MAF sensor and all in front of the condensor along with the IAT to see if that would work but cutting of the core support would be involved and I'm not sure this is a big enough issue to do that yet.

Now on the air lid base question of cut vs un-cut, I'm going to do a back to back comparison in similar temperature weather to see how much an open air lid base does effect the IAT. My open base is kinda worse than stock because the plastic shield that covers the condensor/radiator assembly is completely gone on my car, so when sitting still I am getting radiator heat directly up into my air lid. Maybe one of the scoops alot of guy's build might help too???

[GregRexAdams]

Anything to block a direct path for the hot air, maybe even a dropped and slanted flat panel, that provides more room for air in the box but still blocks a direct upward travel of heat.

[LSxpwrdZ]

I swapped air lid bases tonight with a stock one that hadn't been cut on to see if that helps any. I've just also learned that my dad has some liquid ceramic paint like stuff that came from NASA that goes on the bottom of the space shuttle... he said alot of people use it for insulating walls and things in buildings now and 1/8" is equivalent to R-19 insulation and it's flame retardant. I'm thinking of making some sort of plastic bezel to go under the lid base with this stuff on it and also applying this stuff to the lid base and the lid to see if that helps keep heat out as well. If this ends up working, I am going to provide a service for applying this stuff to different parts.

Only bad thing is this stuff is a little over $100 a gallon!