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Odd... Car runs great warm, but not cold

Old 05-16-2019, 08:25 AM
  #1  
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Car: 1988 IROC-Z
Engine: 5.7 TPI
Transmission: 700R4
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Odd... Car runs great warm, but not cold

My 88 has an issue with running cold. It is odd, I can find 1000's of threads about issues where it runs cold and shuts off warm or won't re-start warm, but not the opposite. This issue I have is, when I first start the car, it will idle up around 1K RPMs and after 15-20 seconds or so it will start to stumble and die if I don't feather the throttle a bit... then it comes right back up, and will idle for a while and then... start trying to die again and will if I don't feather the throttle again.

After a few minutes of this, if I try to drive it out of the garage, it struggles horribly. Almost like the TPS is reading right. If you open the throttle it won't do anything to try and accelerate the car. Just lugs around, but actually doesn't die. Put it back in neutral, and then you can rev it up, but put it back in D and it goes right bag to lugging around. After a few laps around the neighborhood, it then will run normally and I can go out and drive.

It has a re-man ECU
I had replaced the MAF relays and MAF to get rid of the code 36 I was getting (Now has Bosch)
New IAC in the last year (about 500 miles) and minimum idle reset procedure performed. When warm it idles in P between 500-600RPMS and about 800 or so in P.
Code 34 previously corrected as well... had part of the supercharger piping break - has been repaired.
Previously had a gas leak from the O-Ring to the 9th injector.

It did sit over the winter and does have old gas. It has Stabil in it, but the gas may have been 3 or 4 weeks old when I treated it. It wasn't the plan, just some other things got in the way and I had it up on jack stands for a brake overhaul as well. I did put about a half tanks of fresh 93 in it last night.

I know I need to get some data logs, but while I am logging what should I be looking into?

Car has a supercharger kit with an MSD 6AL as well as an MSD boost retard box, headers with no AIR and about 62K miles.

Last edited by KyleF; 05-16-2019 at 11:25 AM.
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Old 05-16-2019, 11:00 AM
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Re: Odd... Car runs great warm, but not cold

Since I never let anything go, I guess I can't tell you to search for threads with symptoms of failing injectors.
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Old 05-16-2019, 11:14 AM
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Car: Two 1987 Iroc-Z Z28s
Engine: 5.7 TPI ZZ4 1.6 Roller Alum. Head
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Re: Odd... Car runs great warm, but not cold

Things to check - Coolant temp sensor reading off 20+ degrees cold. MAF is the only fuel control cold until the O2 sensor becomes active and fine tunes fuel trims. Check for debris in front of MAF like from a mouse or insects.
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Old 05-16-2019, 11:34 AM
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Re: Odd... Car runs great warm, but not cold

Originally Posted by travsirocz View Post
Things to check - Coolant temp sensor reading off 20+ degrees cold. MAF is the only fuel control cold until the O2 sensor becomes active and fine tunes fuel trims. Check for debris in front of MAF like from a mouse or insects.
Oh, I should add I did replace the O2 last summer as well. Has a Bosch unit.

Doesn't seem to have issues when it is warm. I will check round the MAF, but shouldn't have anything in the way. It seems to suck air in just fine once out driving, but it only takes a minute to look. I will see what the ECM reads from the CTS at start up.
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Old 05-16-2019, 11:42 AM
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Re: Odd... Car runs great warm, but not cold

All it takes is there to be something in front of the MAF that disrupts the airflow which skews the reading. I don't think something is blocking the airflow. If you have a scanner, what are the fuel trims?
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Old 05-16-2019, 12:53 PM
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Axle/Gears: 3.70
Re: Odd... Car runs great warm, but not cold

Originally Posted by travsirocz View Post
All it takes is there to be something in front of the MAF that disrupts the airflow which skews the reading. I don't think something is blocking the airflow. If you have a scanner, what are the fuel trims?
I have WinALDL and a MOATES cable.

My only concern there is there may not be enough miles logged since the last time the battery was unplugged to get a full picture. I plan on cleaning on her tonight and doing some scans. The good news is it is very reliable that is has issues for the first few minutes before driving. So, I should be able to get some comparison logs.

Going to also get some fuel pressure readings.

Last edited by KyleF; 05-16-2019 at 04:07 PM.
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Old Today, 07:51 AM
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Car: 1988 IROC-Z
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Axle/Gears: 3.70
Re: Odd... Car runs great warm, but not cold

Did some diagnostics:

CTS was showing appropriate readings. When I started it was showing 107 and warmed up as I let the car run. I had driving it earlier in the morning so I feel this is appropriate even if not 100% spot on. The intake and all was still warm to the touch.

TPS Volts was .53V at idle and 4.63V at WOT (Using pedal and WinALDL) so this is what the ECM is actually reading.

Checked Fuel Pressure:
KOEO: 42PSI
Idle (W/Vacuum): 38PSI
Idle (W/O Vacuum): 46PSI

Leak Down
0 min: 42PSI
5min: 34PSI
10min: 25PSI
15min: 22PSI
20min: 19PSI
25min: 18PSI
30min: 16PSI

Also, I observed only a few cells of 8 or 12 knock counts from starting up.

I noticed the IAC was sitting at 160 and staying solid while it was struggling. So I gave the minimum throttle screw two turns out and then a half a turn back. When Idling around 1000RPMs the IAC is now in single digits.

I will need to re-adjust the TPS. It is now sitting at .69V at idle. I don't think this is much to be concerned about, but no reason not to get it set right.


Any ideas what I should be digging into next?

I didn't take the MAF piping apart yet.
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