1994 Mustang 5.0 - rpms drop when pressing gas while stopped.

Niketay

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Jun 28, 2018
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Hey all, just joined today. I have a 1994 Mustang GT 5 speed.Recently have Been having idling issues which turned out to be the IAC. No big deal, fixed it. Everything runs smooth except when starting from a stop. The car idles at 900rpms and then drops down to below 500rpms and then accelerates. Maybe its fuel related but I’m not too sure. Once it’s rolling and driving at any speed it seems to be fine. I will note though, when going any speed, be it 10 or even 70mph, when you push the clutch in, and let the rpms settle, it still drops a little before it picks up but not as bad as when stopped. It doesn’t seem to starve for fuel even at rev limiter so I’m unsure as to what it could be. Plan on changing fuel filter here soon to see if that will help.
 
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jozsefsz

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Aug 11, 2013
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Does sound maybe fuel-related to me too, check the fuel pressure (which is actually lowest at idle) and you'll know. Ignition timing (and other usual tune-up items like plugs, wires, cap & rotor) can also cause hesitation. As can vacuum leaks. And unfortunately about a thousand other things on a 25 year old probably-modified car.
 

Niketay

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Jun 28, 2018
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Georgia
Very true. I got around to changing the filter today. Haven’t really noticed a change honestly, but for a few bucks couldn’t hurt. The car is driveable, just use a heavier foot and it does just fine. Once it’s moving you can even just barely toe it, and the rpms don’t drop. It’s just when it’s at a dead stop and you barely barely touch the pedal the rpms drop then go up then settle. Using a more forceful step from the idle seems to work fine. It’s just that one small issue that bothers me. Sometimes if you catch it just right, and press lightly and slowly, it will just stall out and die. But as I said before, completely drivable with a heavier foot. I plan on replacing the ICM and distributor here soon because my current one has a lot of miles on it and I feel like it misses just a touch. I’ll keep this updated, so when I find the solution, I can let any other SN95 5.0 owner know what I did. Any other suggestions are completely welcome and thank you Josefsz for telling me some things to check out!
 

Niketay

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Jun 28, 2018
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Georgia
Sorry for the delayed response. I bought an Innova Digital Scanner for OBD1 Fords. Running the KOEO test brought up 2 codes for missing EGR stuff because it was deleted when i had bought it so I don't think that should be a big deal. The KOER test only threw back code 157 for "MAF below minimum voltage". Any ideas if its the MAF screwing up? I'll try cleaning it and probing some wires to see what i can find.
 

Chythar

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Aug 26, 2004
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Sorry for the delayed response. I bought an Innova Digital Scanner for OBD1 Fords. Running the KOEO test brought up 2 codes for missing EGR stuff because it was deleted when i had bought it so I don't think that should be a big deal. The KOER test only threw back code 157 for "MAF below minimum voltage". Any ideas if its the MAF screwing up? I'll try cleaning it and probing some wires to see what i can find.
Yup, I got a 157 code when my MAF died. They're pretty cheap these days, so it's worth replacing and see if it helps.
 

Niketay

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Jun 28, 2018
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Georgia
So while waiting for the new MAF to get here, I decided to leave the current one unplugged and drive around a bit like that. It drives a little different but doable. Still get the idle surging. My idle seems to surge downward like the engine is just trying to catch itself. Still will die sometimes when taking off from a stop. Could it also maybe be a timing issue?
 

Niketay

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Jun 28, 2018
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Georgia
Also forgot to mention it looks like the EGR and Air pump are both deleted. I don’t even see the vacuum lines for them anywhere, so maybe a vacuum leak?? Is there a way to test for that? Sorry for double post.
 

Niketay

Member
Jun 28, 2018
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Georgia
I’ve changed the MAF sensor and the car does idle better for sure. Code 157 gone. Car drives fine under normal throttle now. The only time it dies now is when you stomp on the throttle all the way to the floor in 1 second. It sounds like a tap happens when you do it, almost like all the air coming in just stopped. I tried to recreate it while under the hood and pulling the throttle cable and was able to do it. The sound comes from the area around my air filter on my CAI. Any ideas?
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Pull the air filter off and try to recreate it, if your air filter is in the fender cavity like it should be, don't drive it very far, you don't want dirt getting in the maf.
 

Niketay

Member
Jun 28, 2018
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Georgia
So I’ve figured out my problem! Changed the Distributor, Ignition Coil, and one spark plug wire. After the distributor and coil were replaced, definitely ran better but still had the misfire when warm. As I was going over everything with the ignition system, I noticed the number 4 spark plug wire boot was touching the header! When i pulled it off, the boot was covered in this white powdery substance ( maybe insulation that was burning off?) then put in a new one and turned the boot upwards off the header. No longer bogs down when first pressing the gas pedal and revs smoothly with no hanging idle! Was a simple problem but man did it take a long time to figure out. Also no longer running lean (maybe the code was thrown because of misfire??). Thank you to everyone who has helped me out with suggestions! These forums have potentially saved me so much money.
 
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