Idles great, but stumbles when driving

Stang9050 · Oct 15, 2025

I'm the original owner of a 1990 LX 5.0 with 129k miles. Car stumbles while driving (also in lower gears). Car is slow to start, but idles normally. I did vaccum check and checked for codes (with new OBD1 scanner) and no codes. Wondering if it's a ECT sensor, or ACT sensor?

gkomo · Oct 15, 2025

No codes or code 11, 10? Both KOEO and KOER?

Stang9050 · Oct 15, 2025

Koeo or koer gives code 11 after scanning.

AeroCoupe · Oct 15, 2025

The ECT or ACT would trip a code. Have you checked the MAF for debris on the element? Have you done a tune up on the car recently (plugs, cap, rotor, plug wires, and fuel filter)? If not it may be a good idea to look these items over and do one. You say it is slow to crank, have you check the battery and the voltage at the battery when the car is running?

Stang9050 · Oct 15, 2025

Fresh tune up 6 weeks ago. Rechecked timing yesterday . It just started messing up 5 days ago while driving. New maf 3 months ago along with tps. Until the other day it ran well and started immediately. I've had ect sensor go bad in OBD2 and not give a code.

Mustang5L5 · Oct 15, 2025

Bad ECT/act and it likely would not restart when hot due to flooding out.

Does it stumble in tip in? How new are your o2 sensors? Are you sure you are firing on all 8 cylinders? All primaries hot?

I’m suspecting a fueling issue (lean). I’d check for a misfire. At the end of theOBD1 code reading process, you can blip the throttle and start the cylinder balance test. It’s a crude way to test all 8 cylinders to see if the injectors are firing and the spark is igniting.

Wouldn’t hurt to test fuel pressure either to make sure you aren’t going lean when you roll on the gas

Stang9050 · Oct 15, 2025

Thanks I'll try the cylinder balance test. The stumble happens whether cruising, or accelerating

General karthief · Oct 15, 2025

Read fuel pressure

Noobz347 · Oct 15, 2025

+1 to checking fuel press and 'tip in' stumbling makes the TPS a suspect (even if it is new). Disconnect, clean, and re-connect the TPS. Don't concern yourself with [Key On] voltage.

Also: Idle up to operating temp (at night) and check for arcing under the hood and around the coil.

Stang9050 · Oct 17, 2025

I did the cylinder balance test twice and scored 90 each, but now, koeo, I'm getting code 66 for maf low voltage. I replaced it and still getting that code. I'm not good with electrical wiring, is it time to seek professional help? Thanks for all the responses, I really appreciate the help.

AeroCoupe · Oct 17, 2025

Good read:

Thread 'Code 66 Help!!!'

Oct 6, 2017

All,

I have a 90 Gt that I have completed a top end rebuild on. All new gaskets and hardware. When I got the car done it had a hell of a surging idle. Cleaning of the 10 pin connectors finally tamed that down to an acceptable level. The car has always had a code 66 for MAF voltage low since I have had it.

Here is the issue. The wiring for the MAF is good and shows no resistance to the computer. The signals all check out based on the surging idle checklist numbers. The power and ground are good. This is a new MAF stock replacement. The one on the car when I got It was also a stock unit. I...

Stang9050 · Oct 17, 2025

AeroCoupe said:
Good read:

Thread 'Code 66 Help!!!'

Oct 6, 2017

All,

I have a 90 Gt that I have completed a top end rebuild on. All new gaskets and hardware. When I got the car done it had a hell of a surging idle. Cleaning of the 10 pin connectors finally tamed that down to an acceptable level. The car has always had a code 66 for MAF voltage low since I have had it.

Here is the issue. The wiring for the MAF is good and shows no resistance to the computer. The signals all check out based on the surging idle checklist numbers. The power and ground are good. This is a new MAF stock replacement. The one on the car when I got It was also a stock unit. I...

BossmanPJ

Replies: 12

Forum: Fox 5.0 Mustang Tech

The ten pin connectors, I assume that is the salt and pepper shakers. Mine had a factory recall shortly after I bought the car, now it two sets with the second one spliced into it. What lube do you recommend.

nickyb · Oct 17, 2025

I like the lube that gets warm, wait you mean for the ten pin connectors.

AeroCoupe · Oct 17, 2025

Easy this is my area of expertise or at least my stupid CT says so.

Remove the second set of connectors and toss them. Depin the original ten pins and close the female pins a bit and expand the male pins. Now they will connect better. Dielectric grease is your friend. I did this to the ones on my 93 and still have Ford’s fix in a drawer in the garage.

nickyb · Oct 18, 2025

Agreed Aero, Ford's bandaids suck. Heck I tossed my quad shocks as soon as I got my upper and lower rear control arms upgraded and have never had wheel hop again.

General karthief · Oct 18, 2025

Stang9050 said:
What lube do you recommend.

OHHH, so new and unknowing........

Stang9050 · Oct 30, 2025

So, here I am again. Cleaned and lubed 10 pin connectors, changed fuel filter. I put new spark plugs in and discovered only plugs 4 & 8 are normal all of the rest shows very lean condition. I went to AutoZone for the loan a tool program and got the fuel pressure test kit. I hooked it up and didn't get any pressure readings even when I started the engine. Car runs well until it starts to warm up, then it jerks, bucks and stumbles. I don't understand the zero reading,, it's a new pressure tester.
Thanks for all the help.

rwordenjr · Oct 30, 2025

Stang9050 said:
So, here I am again. Cleaned and lubed 10 pin connectors, changed fuel filter. I put new spark plugs in and discovered only plugs 4 & 8 are normal all of the rest shows very lean condition. I went to AutoZone for the loan a tool program and got the fuel pressure test kit. I hooked it up and didn't get any pressure readings even when I started the engine. Car runs well until it starts to warm up, then it jerks, bucks and stumbles. I don't understand the zero reading,, it's a new pressure tester.
Thanks for all the help.

You must not have hooked up right. Car wouldn't start if you had 0 fuel pressure.

Mustang5L5 · Oct 30, 2025

Some testers out there have trouble depressing the schrader on the fuel rail. That’s likely what’s happening here. Make sure the tester you are using has the feature on the end to press down the little nipple inside

Stang9050 · Oct 30, 2025

rwordenjr said:
You must not have hooked up right. Car wouldn't start if you had 0 fuel pressure.

That was my thought, but when I took the gauge off there was fuel. The kit requires using adapters (included) to attach to the shrader valve and I used a wrench to tighten it before attaching the gauge to the adapter. I even come in the house to watch YouTube to make sure I doing it right.

Idles great, but stumbles when driving

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now i can hopefully expect to receive the shaft

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lube between the nut and the face. I know my lubes

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That is…until I whipped out my Bissell

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wonder how much it would cost to ship you a pair

Stangnet Facilities Maint Tech... Er... Janitor

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lube between the nut and the face. I know my lubes

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I gotta say i never painted my nuts, Never Ever

lube between the nut and the face. I know my lubes

I gotta say i never painted my nuts, Never Ever

wonder how much it would cost to ship you a pair

Member

Advanced Member

That is…until I whipped out my Bissell

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