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Dan
Dan, ASE Certified Technician
Category: Ford
Satisfied Customers: 1650
Experience:  Auto Technician
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What would cause a 95 mustang gt to run a little rough at

Customer Question

What would cause a 95 mustang gt to run a little rough at idle and hesitate when hitting the gas for a few minutes until it warms up?
Submitted: 11 months ago.
Category: Ford
Expert:  Lou P. replied 11 months ago.

Hello, this sounds like you may have a vacuum leak or a failing coolant temperature sensor, the engine must run a little rich when cold so if there is excess air the engine will not run as crisp, also if the coolant temp sensor is stuck more to the hot side then it will make the ecm run more to the lean side which will give you the same effect. there is a very slight chance it can be because of the mass air flow sensor and the tubes you , so if all else checks out fine you may want to see if a different angle of the tubes helps direct flow across the air flow sensor properly. i had a personal experience where the air flow at low speeds was not passing directly through the mass air flow sensor so it made the ecm run lean causing misfires for no reason. hope this helps, Lou P.

Customer: replied 11 months ago.
How do I check the coolant temp sensor? I replaced it a couple years ago.
Customer: replied 11 months ago.
Also, it has a Mountaneer 5.0 block/heads/intake with my original H.O. cam and springs. I haven't done any computer tuning.
Expert:  Lou P. replied 11 months ago.

ok you will have to ohm the sensor at different temperatures to see if it matches the chart. ill try and get you the chart .

as for the engine swap it should not matter that much. if it did the engine would run like that no matter the temp i would assume. so lets rule out the other items first.

dont forget about the vacuum leak.

Expert:  Lou P. replied 11 months ago.

here is the chart

Expert:  Lou P. replied 11 months ago.

also a sticking idle air control valve would also cause a vacuum leak fyi, just an internal leak.

Customer: replied 11 months ago.
I "ohm'ed" the sensor and get 14.2 with my meter set to 200k. It is about 97 degrees outside today. The car was a little healthier today, maybe because it's so hot outside
Customer: replied 11 months ago.
I drove the car for about an hour, mostly on the highway in 100 degree heat (not sure if that matters). When I parked it, it stuttered and died. It would start back up, but hesitates and runs rough, then eventually dies. I'll let it cool off and see how it does later.
Expert:  Lou P. replied 11 months ago.

My apologies I will have to opt out to let another expert assist. I will not be able to respond until Sunday and I do not want you waiting on me. Lou

Customer: replied 11 months ago.
Ok, thanks
Expert:  Dan replied 11 months ago.

do you have the gt40 or gt40p heads?

Expert:  Dan replied 11 months ago.

are you using the mustang ho cam firing order?

Customer: replied 11 months ago.
Gt40p
Customer: replied 11 months ago.
HO cam with "stock" distributor/firing order
Expert:  Dan replied 11 months ago.

did you have problems with the spark plug angle?

Expert:  Dan replied 11 months ago.

what firing order are you using, the 137 or the 154

Customer: replied 11 months ago.
Stock 95 H.O. firing order is how it is set up
Expert:  Dan replied 11 months ago.

what did you set the base timing to on the distributor?

  • 5.0 HO Firing Order: 1-3-7-2-6-5-4-8
Expert:  Dan replied 11 months ago.

did you use a timing light?

Customer: replied 11 months ago.
Plugs are very difficult to access. Base timing is now at 20 at idle, but rolls back to 18 when reving it up
Expert:  Dan replied 11 months ago.

I just got done rebuilding a 1995 302 ford it was for a boat tho, but same thing.

Expert:  Dan replied 11 months ago.

are the plugs hitting the manifold?

Expert:  Dan replied 11 months ago.

20 at idle seems like alot of advance at idle. I set mine to 10 at idle.

Customer: replied 11 months ago.
I used a timing light. Plugs do not his the long tube headers that the car had when I got it
Expert:  Dan replied 11 months ago.

nice. I heard alot of people having problems with the location of the plugs on the gt40p heads

Customer: replied 11 months ago.
I read the p heads like 16-18 base and when I did that the cat made a lot more power. I run 91 and have never heard it ping
Customer: replied 11 months ago.
Car, not cat lol
Expert:  Dan replied 11 months ago.

I have a 351w cam and 351w heads on mine, it makes a ton of torque, but its a marine engine.

Expert:  Dan replied 11 months ago.

does your heads have the egr holes in them?

Customer: replied 11 months ago.
Nice... egr holes? Don't think so. The stock e7 had holes in the back for the smog pump
Expert:  Dan replied 11 months ago.

I think i'm thinking about the gt40 heads not p. I dont think the p heads have the holes that need to be plugged.

Expert:  Dan replied 11 months ago.

you should put a vacuum gauge on it and see what it does when its running rough.

Customer: replied 11 months ago.
I get about 20 pounds or so at idle
Expert:  Dan replied 11 months ago.

do you think you have a misfire when cold?

Customer: replied 11 months ago.
Seems like it has a misfire at idle, especially when cold. Hesitates bad when cold.
Customer: replied 11 months ago.
Even when it's at its best, ***** ***** it should make a little more po we r. You have to be baby it, say, at low speeds, turning, maneuvering in a parking lot or it will die
Expert:  Dan replied 11 months ago.

have you tried cleaning the throttle body and mas air sensor

Customer: replied 11 months ago.
Expert:  Dan replied 11 months ago.

did you check compression?

Customer: replied 11 months ago.
Haven't checked compression, really don't want to
Expert:  Dan replied 11 months ago.

its alot of work.

were you able to get any codes?

Expert:  Dan replied 11 months ago.

what it you have a stuck piston ring, then when the piston warms it expands then seals.

Customer: replied 11 months ago.
Had codes pertaining to emissions but no missfire codes or anything like that. It was a while ago.
Customer: replied 11 months ago.
I'd like to run a cylinder balance test, but last time I tried, it wouldn't cooperate
Expert:  Dan replied 11 months ago.

while its running rough, pull a plug wire off the dist cap one at a time and listen for an rpm to change. use a pliers with rubber handles and wear gloves.

Expert:  Dan replied 11 months ago.

that way you can narrow down which cylinder is missing. then focus on that cylinder.

Customer: replied 11 months ago.
Absolutely great idea. I will do that tomorrow so I don't wake everyone up. I'll get back with you afterwards, I'm sure your curiosity is pegged!
Expert:  Dan replied 11 months ago.

I am here to help, you got the best ford 302 tech on here lol

Customer: replied 11 months ago.
Right on, thanks man
Expert:  Dan replied 11 months ago.

Let me know what you come up with.

Expert:  Dan replied 11 months ago.

Hello again Dan here,

I am contacting you because the post has not been closed or rated. We have 7 days to get a post rated before it times out. When a post times out, that means its no longer on my active list (if you reply I never know it.) You have already paid the fees to get this question answered, but it appears you’re not pleased, or need more help.

Let me help some more. Is there anything else you feel I could offer so you would be pleased with my help.

Thanks Dan

Expert:  Dan replied 11 months ago.

Dan again, hoping you are satisfied with my service.

Could you please rate the answer, it would be greatly appreciated. Thanks!