89 GT stumbles 2500 and above

Dtambs39

Member
Feb 25, 2021
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Dallas
Hello

I am new to this forum and I have an 89 Gt 50K original with AOD and it has a a problem that I can't seem to figure out. I have done a full tune up, including all electronics on and in the dizzy including wires. Replaced the fuel pump, sock, filter and regulator. Set timing to 15 BTC. Changed every sensor on the motor including MAF. I had the 3 capacitors in the ECM professionally changed out, that gave it more power but still stumbles. Changed out the upper intake gasket, egr, IAC, O2 sensors and throttle body gaskets. Changed out the pcv and hose to it. Have also changed out all the injectors to new 19lbs. Have done a smoke test for vacuum leaks. I found none. Even checked by spraying all vac lines with starter fluid to see if rpms will jump, no dice. The car starts no problems and idles pretty good. You can drive it regularly up to about 2500 and its just fine. At WOT from a start its fine till it hits 2500 and above almost like a fuel starvation or timingissue. So far nothing that I have done has changed the stumbling issue, Need some help.

Thanks
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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That's both ends of the spectrum, now test it in the area of the problem, is the rise in voltage steady?
Here, go through this checklist, it may help eliminate things.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
This situation rang a bell for me so I went searching, what I found was not really similar to this but begged some questions.
Where is the timing set at?
What brand cap, rotor and spark plugs?
Is your fuel pressure steady throughout the rpm range?
 

Dtambs39

Member
Feb 25, 2021
23
2
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Dallas
That's both ends of the spectrum, now test it in the area of the problem, is the rise in voltage steady?
Here, go through this checklist, it may help eliminate things.
The voltage I checked when the car had just the key on to power it up. It seemed to sweep fairly steady. Should I just change out the TPS anyway?
 
Last edited:

Dtambs39

Member
Feb 25, 2021
23
2
13
50
Dallas
This situation rang a bell for me so I went searching, what I found was not really similar to this but begged some questions.
Where is the timing set at?
What brand cap, rotor and spark plugs?
Is your fuel pressure steady throughout the rpm range?
The timing I have set at 15 degrees BTC, rotor and cap I am not sure it came from rock auto. Its not a motorcraft that I know. I have not tested the fuel pressure yet. I have replaced the regulator, stock style. The fuel pressure was something that I was thinking of. But with all the new items that I put in the fuel system you would think that something would change either better or for worse.
 

HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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My car acted like this when the IAT sensor was unplugged and when my fuel pump was going bad.
 

Dtambs39

Member
Feb 25, 2021
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Dallas
My car acted like this when the IAT sensor was unplugged and when my fuel pump was going bad.
Well my fuel pump I had replaced with a new one about a month ago just because, still no change in the stumble. The IAT I changed it out probably 4 months ago. No change either, The old one was really dirty. I will recheck the connection tomorrow.
Thanks
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
Ok, I would verify the fuel pressure is good through out the rpms, verify the tps sweep is smooth between idle and wot, set the timing at 12*
I don't care if you replaced this or that, you have to verify things operate as they should, the shop I hang at has a terrible time with 'new' replacement parts that malfunction sometimes worse than the part it replaced.
You have to be methodical and not assume something is doing what it is supposed to do and not just 'I replaced that already so it's good'.
Going through the checklist I linked step by step will help you out tremendously,
 

Dtambs39

Member
Feb 25, 2021
23
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Dallas
Hello
I have a fuel pressure tester on order. I reviewed the link you sent. I went through the salt and pepper connectors today opening the pins a bit along with degreasing and adding new dielectric grease to them. No change today.

Best
 
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Dtambs39

Member
Feb 25, 2021
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Dallas
Question how do I see the fuel pressure on the tester if its just 18" long while driving? Its arriving today but I thought about that last night.

Best
 

Dtambs39

Member
Feb 25, 2021
23
2
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50
Dallas
No gauge yet, the one I ordered was a betool kit.
So updates on testing MAF holds .6 volts at idle, on ohms it only 3.97 I read that it should be 10 correct?
On the ECT Sensor it read 5.99 resistance
ACT holds 4.4 volts at about 120 degrees.
Located 3 grounds that I removed sanded to metal and reattached.
All the sensors I opened the connectors cleaned and applied dielectric grease.

I repost when the fuel pressure gauge shows.

Best
 

Bree

Active Member
Jan 12, 2021
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TPS are cheap. Replace it anyway, because the described problem screams TPS. Did you just do a voltage test on it, of did you do a resistance test with an analog meter? The drop out in a TPS is sometimes very subtle. You watch the numbers climb, and climb on a digital meter and the quick hit on the dropout, can happen so fast, you never see it flash to 0, it just looks like the flash to the next higher number. With an analog meter, as you watch the needle climb, you will see it twitch at the drop out.