Can someone please review a data log for me? This post explains my issue in detail. But the long short of it is the car (93 M6 in sig) does not want to idle when cold started. It goes from running smooth for 10 seconds or so when started cold to suddenly running really rough, and rich (stinks of gas), wants to stall. I have to use the throttle to keep it alive until it warms up. It runs/idles fine in closed loop. If I unplug the TPS sensor, it will idle high (1500-ish rpm), but smooth until warm up. In the attached data log (from Scan9495), I'm using the gas to keep the car running until warms up.

In my very limited/new knowledge of understanding data logs, the IAC (new Delphi) counts seem whacky/high. I reset it both with the pedal and Scan9495, didn't matter. Among its bolt ons, the car has as tune and an air foil, which I read could impact IAC numbers.

Thank you in advance for any help. I've been trying to figure this out since summer.

 

Are you getting any codes? Have you done anything to the vehicle before the issue started, like tune up or upgrades?

 

No codes. The thread I linked to goes through all the troubleshooting and parts I've thrown at it. The main thing was the car ran fine, then I let it sit for years.

 

What was the outside temp when you took the log? The intake air temp looks screwy to me. I'd take a close look at the IAT sensor/wiring. When the thing is idling steady, it's double the target. I think that says either vacuum leak or the ECM is calculating the wrong air density.

On one of my idle logs, my air intake temp only varies only by 5*(63-68*) from an engine temp of 77* to 172*F. Your IAT is showing swings from 34* to 97*F with an engine temp of 30* to 180*F. Given the temp started out at 30*, I'm going to bet it was a cold day when you took the log. If so, the IAT should have stayed around that value give or take a few degrees.

I also wouldn't rule out some kind of ignition or fueling issue as the thing is running so rough at times. Especially given that it sat for a long time.....might have screwed up the injectors and how they flow. So, looking over the plug waveform firing lines and injector flow is where I would start after getting the IAT sorted out. Scanner Danner's youtube site is an excellent resource on how to do this right. As usual with an LT1, run the opti checks as highlighted in the Buick Know How video.

Injector balance testing....

Opti diagnosis...

 

Interesting on the IAT. I had swapped an extra one on at some point and it didn't change anything. The wiring for it looks good, but it can't hurt to look again. It was a cold day probably the 30*F initial air temp in the log, like you said; I usually confirm that it's close. My CAI is metal (elbow is rubber)...could that be heat soak? It seems to climb with the engine temps. The car is way harder to start/runs richer for longer when it's cold outside.

I've ran the opti checks and the car checks out fine. I've bench tested all the injectors with a transformer and confirmed they don't leak by priming the fuel pump with the fuel rails pulled and the injectors still attached.

Once the car is warmed up/in closed loop, it runs great, which is weird.

Thanks for your help.

 

Interesting on the IAT. I had swapped an extra one on at some point and it didn't change anything. The wiring for it looks good, but it can't hurt to look again. It was a cold day probably the 30*F initial air temp in the log, like you said; I usually confirm that it's close. My CAI is metal (elbow is rubber)...could that be heat soak? It seems to climb with the engine temps. The car is way harder to start/runs richer for longer when it's cold outside.

I've ran the opti checks and the car checks out fine. I've bench tested all the injectors with a transformer and confirmed they don't leak by priming the fuel pump with the fuel rails pulled and the injectors still attached.

Once the car is warmed up/in closed loop, it runs great, which is weird.

Thanks for your help.

 

The IAT in the factory location is subject to heat soak, as the engine compartment heats up. In a severe case like hot summer day, A/C on, and sitting stopped in traffic the IAT reading was as much as 40°F above ambient air temp. I was still running a Callaway CAI and relocated the IAT to the cap of the cone filter, located in the driver front fender, isolated from the engine compartment. With the Scanmaster LT1 mounted permanently on my steering column, it showed ambient air temps at all times.

What I should have done is briefly check the stock location readings compared to the CAI readings, and I would have had a measure of the heat gain through the CAI duct in the engine compartment. Never did that because the IAT reading is relatively unimportant running the MAF. But in speed-density, the more accurate the better.

 

Just tossing this out there....

Are you sure the engine is mechanically sound? I had a 3.1 V6 that had a crack develop in the cylinder and would run rough until warm. The crack would close up as the engine got hot. It lessened the coolant flow being pulled into the cylinder to allow it to run mostly like normal. From what I understand, the 3.1 V6 had a history of cracking between 1 and 3, but I've never seen cylinder cracking report issues on the LT1. Head gasket could an issue....but I'd lean towards checking things that could happen to a car after sitting for a long time. Especially looking over the injectors and opti components.

Summer is coming up, I can get some readings on my IAT with it on a hot summer day. If it's nice this weekend, I'll get some new idle readings on it.

 

Thanks on the IAT readings. It's a good thought, but I think the engine is probably fine. The engine/car was totally sound when I parked it. It's always had regular oil changes/been well maintianed since I've owned it for the last 20 years/45K miles (aside from neglecting/not driving it). Any time it sat, I've changed the oil before driving it anywhere that wasn't around the block. There is no coolant in the oil or oil in the coolant.

Opti checks out, injectors pulse when bench tested and don't leak. Fuel pump and GM opti are both pretty new, fuel filter is new, main fuel line is new, fuel pressure regulator is new. I've replaced any vacuum hoses that were starting to dry rot/all the gaskets on/in the throttle body. And while I'm listing all the new parts I've thrown at it/swapped in extras: replaced a stuck EGR Valve, an EVAP solenoid that didn't hold vacuum, MAP, IAC, TPS, IAT, coil, wires. I'm sure I'm forgetting things. So if anyone wants a cold start mystery 93 z28 with its bluebook value in new parts installed...j/k it's not for sale.

Yesterday it was 70*F out. The car was hard to start as ever, so maybe there is no correlation to temperature. It is worth noting, when I replaced the IAC, and TPS at different times and started the car, it started and ran fine the first time after install. So I thought the issue was fixed...until next time I went to cold start the car and the issue still existed.

 

Can someone confirm if unplugging a sensor like the TPS will put the vehicle into limp mode? Also what exactly happens in limp mode?

 

It will cause the PCM to set a code, turn on the SES light, and default to a backup approach, or alternate method of running the engine. Probably will not run optimally, but it will still be drivable. But that isn’t “limp mode”. Limp mode is a default the PCM uses when it has lost most of its communication with the outside world. It's going to run just well enough to get the vehicle off the road, to a safe location. Most people never experience limp mode.