[94FiveOVurt]

Hey guys,

My 94 Mustang GT runs very very bad when its hot outside and has been doing this for a couple weeks now. The temp today was only 70 degrees and it ran very well and the temp gauge was reading normal as well. I also run pretty close to the H on the gauge when its hot outside which is contributing to my issues but could running a bit hot really have that much affect on it?

I've cleaned the MAF, IAT, IACV, new Air filter, New fuel filter, new spark plugs, new ECT sensor, new upper radiator hose, new 192 thermostat. When its running hot in hot weather the car can barely make it out of first gear since it has a huge lack of power and seems to be throwing LEAN codes (172 and 189). What should I look into to get this fixed?

I know I probably need to flush my coolant as its brown in color so that may help the running hot part but not sure what could be causing it to run lean an crappy when its hot outside.

[94FiveOVurt]

Oh yeah forgot to add that its a 94 5.0 5-speed with the following:

BBK offroad H-pipe (catless)
Flowmaster 2.5" catback exhaust
Shorty headers
BBK Throttle Body 75MM
80MM MAF meter
CAI
Ford racing cam

[8850d]

I think it could be not 100% positive to but check you're egr valve. BUT what are the symptoms is it a no power shacking stuttering cutting out wanting to die? Please explain more symptoms.

[94FiveOVurt]

Power seems to be somewhat smooth so no jerking/stubble. Just feels like the car has about 90hp and I have to rev up to about 3000rpm and release the clutch to get going but even then it Bogs a bit. Just feels like it has no power. It doesn't do this when its cool outside or the temp gauge isnt almost hot. Flushing the coolant soon to see if that fixes it but I'll check the EGR as well.

[SStang67]

Its your fuel pump. The electronic ones dont always just go sometimes they act up in heat. I had a 1990 that only on hot weather days it would just bog out but when cool ran fine. After I replaced half the car (computer, cats, O2 sensors, TPS, etc) I finally replaced the pump and it never ran better. Also I tested the fuel pressure even when it was hot but always showed normal or a little below normal, but I assure you it is your fuel pump. Not to hard to change, disconnect filler, drop to straps to drop tank and take out sending unit to change pump. Hope this helps.

[94FiveOVurt]

Fuel Pump huh. That would contribute to the LEAN codes for sure. I replaced one in my 2000 Ford Mustang 3.8 V6 which I'm sure is just about the same. That's an expensive part to not know 100% sure though, anyway to know for sure its the fuel pump? Come to think about it, I did fill my tank up today and it ran better so maybe when the fuel pump is submerged in gasoline it runs cooler than it does when its almost empty?!?!

I know it's running lean too cause I pulled every spark plug out and the tips were solid white

[SStang67]

It's expensive if you have someone do it. It's only around $100 for a new pump. Let me know if you end up changing it and if that was the issue.

[PONYMAN]

Originally Posted by 94FiveOVurt Hey guys, My 94 Mustang GT runs very very bad when its hot outside and has been doing this for a couple weeks now. The temp today was only 70 degrees and it ran very well and the temp gauge was reading normal as well. I also run pretty close to the H on the gauge when its hot outside which is contributing to my issues but could running a bit hot really have that much affect on it? I've cleaned the MAF, IAT, IACV, new Air filter, New fuel filter, new spark plugs, new ECT sensor, new upper radiator hose, new 192 thermostat. When its running hot in hot weather the car can barely make it out of first gear since it has a huge lack of power and seems to be throwing LEAN codes (172 and 189). What should I look into to get this fixed? I know I probably need to flush my coolant as its brown in color so that may help the running hot part but not sure what could be causing it to run lean an crappy when its hot outside.

Brown color in your raditator could be rust & deteriation of your radiator .
Just changed mine for an alumium radiator , it runs super cool now.
Fuel pump could be a definite issue too.

[94FiveOVurt]

Yeah I'll start with that coolant flush and see if that helps.

As far as the lean condition, I found a vacuum leak under the intake manifold so that may help it.
Also would running a 75mm TB and 80MM MAF with CAI and no fuel mods cause it to run lean?

[91cobra5.0]

is your maf calibrated for your size injectors, if your maf is miss calibrated (ex: calibrated for 19# and running 24#)maybe its not running as well in hotter wearther since the air is thinner and the maf isnt getting the right reading?? unless its a huge vaccum leak i dont think it would cause that extreme or a situation, the bigger maf than tb will not cause that, i was running a 65mm tb and 75mm maf

[SStang67]

if it was the radiator it would run hot and bog all the time not just on hot days. I'm telling you its your fuel pump. MAP/injectors not calibrated right would also make it run off all the time not just when hot. Only 2 things I would bet money on 1. Fuel pressure regulator (which I never heard of one going) or 2. FUEL PUMP!! If I was a betting man, now would be a good time!

[91cobra5.0]

Originally Posted by SStang67 if it was the radiator it would run hot and bog all the time not just on hot days. I'm telling you its your fuel pump. MAP/injectors not calibrated right would also make it run off all the time not just when hot. Only 2 things I would bet money on 1. Fuel pressure regulator (which I never heard of one going) or 2. FUEL PUMP!! If I was a betting man, now would be a good time!

my fp regulator craped out and it would hesitate but only under wot, i would also agree that the fuel pump could cause the issue. look into a 255 ho walbro

[94FiveOVurt]

Yeah no severe jerking or stumble, more like hesitation or slight miss while at WOT.

Seems to run a bit better now that I fixed a vacuum leak that was under the upper intake manifold, a hairline crack in the FPR vacuum line and replaced upper intake manifold gasket. The big leak was not the PCV plug but the other one (goes from the intake manifold port underneath to the purge canister thing). So now I can actually get out of first gear without bogging like it was but it still has a power issue at WOT.

Is there a way I can test the Fuel Pump at WOT with a fuel pressure gauge?

Also, did a little researching and not sure if this would be an issue too but I have a CAI and my MAF housing is missing the mesh screen which I read could cause the car to run leaner due to the air turbulence not being straightened out by the mesh screen before the MAF sensor reads.???

The car has not ran hot these last few days because the temp has been below 90.

[94FiveOVurt]

Originally Posted by 91cobra5.0 is your maf calibrated for your size injectors, if your maf is miss calibrated (ex: calibrated for 19# and running 24#)maybe its not running as well in hotter wearther since the air is thinner and the maf isnt getting the right reading?? unless its a huge vaccum leak i dont think it would cause that extreme or a situation, the bigger maf than tb will not cause that, i was running a 65mm tb and 75mm maf

Not sure on this as I purchased the car with the mods on it already. It has the orange 19# injectors and I guess I can get the part number off the MAF sensor itself to make sure its not one calibrated for 24's or more.