89 GT 5.0 stumble between 1500-2300 RPM

trip1230 · 10-09-2009, 11:01 PM

I know this is very vague but anyone have any experience with their 5.0 chugging and jerking when holding the rpm steady? I'm talking about when you're just cruising down the street in second or third gear trying to hold a steady speed and the car starts jerking back and forth due to an engine stumble.

I just got it back from the dyno tune shop and they tried to work it out but it still does it. They custom burned a chip for the current setup and were able to pull 371 HP and 418 ft lbs of torque out of it but can't get the thing to run smooth. (this was at 12 lbs of boost and 4800 RPM, it has a stock bottom)

Here's my setup:

Vortech S1 with super FMU
75MM Throttle body
30lb injectors
GT40 upper and lower intake
Edelbrock aluminum heads
Underdrive pulley's etc...
Mass air size is unkown but the dyno shop tested it and calibrated for my injectors. I forgot to ask them but I assume it's a 76MM.

It's going back to them for a re-pull but I thought if someone else has had this problem perhaps you could offer up some ideas?

The guy I bought it from said that he recalled this problem starting around the time he replaced the mass air flow sensor. Could this cause this? IT RUNS PERFECT WHEN COLD.

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mjr46 · 10-09-2009, 11:41 PM

could be ignition misfire

Jerome99 · 10-10-2009, 09:27 PM

Mine does the same thing, it goes away if i get on the gas a little I have been hunting the reason why it is happening since I got the car, I don't think my car likes to go under 40.

STROKED_LX · 10-10-2009, 10:32 PM

Whats the duty cycle on the 30 lb/hr injector? What fuel pressure are you running at? Have you watched the fuel pressure to see if there is any correlation with your issue.

ptown87 · 10-11-2009, 07:43 PM

The fact that it runs good cold makes me think your distributor module could be going bad. These modules are notorious for being susceptable to heat damage. There is a layer of electrolytic paste between the module and the side of the distributor. over time this paste looses it ability the dissipate heat, causing heat damage to the module. Take it off and bring it to autozone they can test it for you. I was having same problems, I got a new one at autozone for around 40 or 50, problem solved.