89' Trans am GTA with 350 tune port engine. Just bought the car and was 'told' it was rebuilt engine with 4L60 auto trans.
Motor starts/runs good...initially. Seems to run a bit hotter than I'd like but... Never over 220-225 but not much below that when in the city. Interstate- 170deg.



I've been experiencing after longer drives (20-30 miles..mostly highway) the car will end up in a reduced power mode and will not allow RPMs over 2-3000. I have to limp home. Seems like timing or spark issue cutting out while driving. When in neutral it will rev but not in gear. Only last night when it did this did I suspect the trans was also acting funny- not sure if there is any relation but in neutral revs good; in gear it will not. Until cools down or restarted.


It developed a low end hesitation which led me to change spark plugs and check EGR- the 32 EGR code flashed once before. 1 plug looked questionable and immediately felt improved but did not stop this reduced power mode after longer drive. I searched and come up with multiple possibilities...timing, rotor, wires, EGR, etc....?


Anyone offer any insight?

 

When the apparent power is reduced is there any noticeable misfire, or does the engine run smoothly but at reduced power?

Do you know if the ECM has a factory program or has it been custom tuned? If the coolant temperature drops below about 176°F (80°C) after sustained operation the factory programming can invoke the default backup fuel and spark parameters. This would result in a noticeable loss of power.

My guess is that either the rebuilder installed a thermostat which is too cold or the thermostat is leaking or sticking open. It's a common error made by those who do not understand the factory ECM parameters and their ramifications. The PROPER method is to install a cooler thermostat AND reprogram the ECM to avoid these pitfalls.

Here are some of the factory parameters relating to coolant temperature:

Coolant Temperature Sensor Related Parameters
BLM enabled between 50̊C (122̊F) and 140°C (284̊F)
Cold spark advance disabled above 56°C (133̊F)
Hot spark retard begins above 116°C (240̊F)
Highway Mode spark advance > 59.8°C (140̊F)
Knock sensor disabled below 66.5°C (152̊F)
Power enrichment at base A/F ratios > 56°C (133̊F)
Target IAC idle RPM >80°C (176̊F)
IAC multiplier at 1.0 (base) > 32°C (90̊F)
Knock Control enabled > 67°C (153̊F)
EGR Duty-cycle enabled at 56°C (133̊F)
EGR Duty-cycle at MAX >80°C (176̊F)
TCC lockup enabled >50°C (122̊F)
SHIFT light enabled >50°C (122̊F)°
Diagnostic communication enabled at 70°C (157̊F)
DTC 43 enabled > 90°C (194̊F)
Cooling fan #1 enabled at 107°C (226̊F)
Cooling fan #1 off at 104.7°C (220.5̊F)
Cooling fan #2 enabled at 115.2°C (239.5̊F)
Cooling fan #2 off at 110°C (230̊F)
Cooling fan duty cycle at 100% at 80°C (176̊F)
Acceleration enrichment multiplier at 1.00 below 80° C (176̊F)
Acceleration enrichment multiplier at 0.75 above 80° C (176̊F)
Acceleration enrichment multiplier at 0.50 above 104̊C (220̊F)
Acceleration enrichment decay factor at 25% below 80°C (176̊F)
Acceleration enrichment decay factor at 40% above 80°C (176̊F)
Acceleration enrichment decay factor at 50% above 104°C (220̊F)
Acceleration enrichment decay factor at 60% above 128°C (262̊F)
Fuel limiting factor timer at 217 counts below 80°C (176̊F)
Fuel limiting factor timer at 169 counts above 80°C (176̊F)
Code 13 (oxygen sensor fault) enabled above 70°C (157̊F)
Code 14 (CTS high fault) enabled above 130°C (266̊F)
Code 32 (EGR fault) enabled above 30.5°C (87̊F)
EVAP canister purge enabled above 70.3°C (158.5̊F)
Hot closed-loop timer enabled above 70.3°C (158.5̊F)
Rich/Lean O² offset at 16 counts between 20°C (68̊F) and 92°C (197.5̊F)
A/C clutch disabled above 150°C (302̊F)

 

As far as I know, stock ecm. Drove again today and experienced the same symtoms. After 20 miles average temp (i did notice this time it got closer to 230deg while in traffic once) before cooling to normal temps (180 deg) while driving.
The car runs smooth put cuts out if too much pedal is pushed and under steady acceration it will cut in and out and lurch- had to get of interstate bc of this. Similar to running out of fuel But as soon as i killed the engine and re fired it was back to normal. And also when in neutral the car would rev fine.

 

This may be a fuel delivery issue, weak ignition from a heat-sensitive coil or HEI module, or (more remotely possible) a fouled cat converter which constricts when heated. It could also be an improperly controlled EGR valve, and likely numerous other possibilities. Verifying fuel pressure would be first on my priority list.

 

Spark control module in distributor can get hot and do the same thing. Also next time you experience this open the fuel cap and see it you have an excess amount of fuel tank pressure.

 

I did notice a lot of pressure released from the gas cap.

 

Also, killing the engine then restarting does correct the problem temporarily.

 

Next time release the pressure without turning off the engine and see if your symptoms dissapear or remain. When are your fans turning on?

 

That's where the remote possibility of an EGR was conceived.

 

What would excessive fuel tank pressure mean. I will check that during the situation next time...
In effort of process of elimination, i removed the EGR system completely and the car does run better(accelerates faster); but did not correct this issue. After some time the symtoms return and it DOES feel like a misfire...the car shakes a bit at idle and more under acceleration.
Whats the best method of testing the spark and fuel pressure?
I assume the engine would have to be misfiring at the time (engine hot!) to check spark...
And considering it is the ECM that invokes this condition, would/could and aftermarket tune correct it?