Stumped - Mustang 5.0 running hot lean

[AGGRV8D]

My Bronco has a '91 Mustang 5.0 w/ Explorer Front, BeCool Radiator, Spal Fan... less than 40K mi since rebuild. Ran fine then started overheating out of the blue, but only at highway speeds (45+). We did a compression test and it was well within range. Replaced Waterpump, Fuel Pump, Thermostat, Overhauled and Pressure Tested Radiator, replaced coil (MSD), ignition (MSD 6AL), all fluids, checked distributor and spark plugs.... Cooling system is working, because it'll drop from 230+ to 200 and continue to fall pretty quickly when I slow down to 25mph. It doesn't seem to overheat under 45-55mph, but isn't running right. Rpm's aren't so high that it would warrant anything like this to occur. It feels to me like its running lean (underpowered, especially at top end, kinda coughs under load...). Going to try and replace MAF sensor today, if that doesn't do the trick not sure what to try next.

Does anyone have any suggestions on what it might be or other diagnostic tools we might not have thought of? Could it be clogged injectors? Clogged Head? Bad ECU? I've already spent a small fortune trying to remedy this, so any advice or diagnostics suggestions would be appreciated.

 

[Broncobowsher]

Lower hose collapsing?

 

[Rustytruck]

What does the plugs look like? I would check the timing curve. But you will need either a dial type timing light or a standard timing light with a timing tape on the vibration damper.

I would be looking for a vacuum issue if your distributer uses a vacuum advance.

 

[Jebus]

When was the last time the HEGO sensors were replaced?

 

[needsmoarturbo]

Something very odd like this happened to me once on a Moab trip. It was running very hot, very crappy at higher rpm and seems like poor mpg. During the trip I diagnosed it as low fuel pressure possibly. I bought a new pressure regulator in Moab and when I went to change it it was like something popped, I heard a noise from it like it came unstuck. Fired it up without changing it and it ran right after that even though I never changed it, so maybe unplug the vacuum line to the regulator and check the fuel pressure both unhooked and with it hooked up. It should be 60+ psi unhooked and like 45 with the hose connected.

 

[Broncobowsher]

That's too high for an early 90's mustang. 43.5 PSI is the operating pressure. Since manifold vacuum will pull fuel harder the pressure regulator is there to keep the pressure balanced between the rail and manifold. The injector only sees 43.5 no matter if there is vacuum in the manifold or not. You should see about 35 PSI with a strong manifold vacuum and about 45 PSI with no vacuum to the regulator.

But this is still assuming it is hot because it is lean and not hot for some other reason. New bumper/winch? Those have been known to block a lot of airflow at highway speeds.

 

[AGGRV8D]

Thanks everyone. I'm going to share your suggestions with my mechanic in the morning. We've both been working on this and are both stumped.
Replaced MAF Sensor and Fuel Pump (again) and it is running a little better. I can get it to spike to 230 at hwy speeds, but not as quickly and not as hot as before (240+ in under a mile). It also drops down under 200 a little faster as soon as I slow down to under 55 mph. I also ran about 1/2 a tank of gas with liqui-moly fuel injection treatment through it, which seemed to help a tiny bit. Everything is helping a tiny bit, but nothing seems to have cured the root cause, whatever that may be. It did run a lot cooler once the sun went down, but it's also 20 degrees cooler outside.
No new bumper, winch or any other mods that would be causing the issue. I honestly think its either fuel starved, an electrical issue (distributor and plugs look okay, but burned up 3 coils past week or so) or some weird vacuum issue. Timing seems to be correct.
Replaced MSD Ignition and coil plus MAF Sensor and Fuel Pump, today which also made it run a tiny bit better.
It does do that weird POP when its cold, think its probably because intake tube is too short and not enough space between MAF and Throttle body. It is something I've been meaning to replace, but has been that way for a while and wasn't overheating. Could also be a vacuum issue (the fox body 5.0's are notorious for), but I don't even know where to start with that.

 

[ransil]

Quit throw parts at it.

Pull codes

check timing Base timing- mechanical- and full timing past experience total advance should not exceed 34 degrees

check fuel pressure while at speed gonna need to rig something up to view it duct tape on the windshield works.

Fuel flow volume gotta look it up cant remember, something like a quart in 30 seconds.

Vacuum leak time to find or make a smoke machine, google it.

not a fan of after market ignition stuff, keep it stock keep it simple.

 

[AGGRV8D]

How do I pull codes? I don't see any OBD or plug on ECU, motor's from a '91.

I'd rather not keep throwing parts at it, but have kind'a reached the end of the rope with diagnostic capabilities. I have already spent thousands of dollars (literally) on "professional" mechanics and the problem is still there. I even tried to convince my mechanic to just toss the motor and drop in a crate motor because the Bill has gotten to that point. He convinced me that this motor is built right and that would be a mistake. Mechanic today told me he couldn't replicate the issue and said it was fine (because he wouldn't take it on highway) then within 10 minutes of leaving his shop I was sending him video if it overheating 2 mi down the freeway. Now he's suggesting a Valve Job will resolve it.

 

[ransil]

How do I pull codes? I don't see any OBD or plug on ECU, motor's from a '91.

I'd rather not keep throwing parts at it, but have kind'a reached the end of the rope with diagnostic capabilities. I have already spent thousands of dollars (literally) on "professional" mechanics and the problem is still there. I even tried to convince my mechanic to just toss the motor and drop in a crate motor because the Bill has gotten to that point. He convinced me that this motor is built right and that would be a mistake. Mechanic today told me he couldn't replicate the issue and said it was fine (because he wouldn't take it on highway) then within 10 minutes of leaving his shop I was sending him video if it overheating 2 mi down the freeway. Now he's suggesting a Valve Job will resolve it.

No mechanic wants to work on thrown together stiff.

Try beach city broncos or find someone a bronco guy or mustang guy that know 5.0L EFI

 

[AGGRV8D]

No mechanic wants to work on thrown together stiff.

Try beach city broncos or find someone a bronco guy or mustang guy that know 5.0L EFI

I won't name names on here, but believe me, I have tried both "Bronco Specialty" and "Mustang" mechanics here in Southern California and continue to butt my head against the wall. Everyone tells me they don't see any issue when it's in their shop but it's definitely there and frustrating me to no end. There's no reason it should be spiking to 240+ degrees in minutes but only at hwy speeds. I can drive it in stop & go traffic on a 110 degree day with no issue but try to go faster than 50 mph when it's 75 degrees out and it spikes. I even replaced the brakes and tore apart the transfer case thinking there was some mystery drag happening.

 

[Broncobowsher]

Spiking that fast, I still say the lower hose is collapsing (sucking shut at speed).

 

[AGGRV8D]

Spiking that fast, I still say the lower hose is collapsing (sucking shut at speed).

Lower radiator hose???
We replaced all the hoses and had the whole cooling system overhauled about a month ago. Including having the radiator blown out and professionally pressure tested.

 

[Rustytruck]

When its overheating does it blow coolant into the overflow bottle?

 

[AGGRV8D]

When its overheating does it blow coolant into the overflow bottle?

I don't have an overflow bottle currently installed, but I don't see any coolant coming from the overflow hose.

 

[BanditBronco]

Pull the thermostat, run it, and see if anything changes. Burning up coils like that though sounds like you have some other issues as well. Next time it won't start because you think the coil went bad, do an actual no start diagnosis step by step, and see what you come up with. The diag how to's are all over the internet for this mustang stuff, just google 5.0 mustang no start and they will come right up.

 

[AGGRV8D]

Thanks everyone: Headed back to my Mechanic with the following list based on all your and mustang experts advice:

• Could absolutely be a vacuum issue, which FB mustang motors are notorious for. Especially if the distributor is vacuum advance (idk if it is). Someone suggested “smoke machine test” and said it’s easy to google or YouTube.
• Could be bad HEGO (O2) Sensors
• Could be low fuel pressure / bad pressure regulator because needs to be balanced between the rail and manifold.
43.5 psi is operating pressure, injector only sees 43.5psi no matter what. Should see 35psi w/ strong manifold vacuum and about 45psi with no vacuum to the regulator
• Check timing - Base Timing - mechanical and full timing past experience should not exceed 34 degrees.
• Check fuel pressure at speed for fuel flow volume

• Google Mustang 5.0 no start diagnostics next time coil goes bad.

I don’t think it’s the cooling system because its been completely overhauled and cools down quickly w/ reduced speed/load, but if it is:
• it could be air in the cooling system, recommend bleeding and/or drilling a tiny hole in thermostat.
• it could be lower hose collapsing under load.
• Pull thermostat and see if it still acts up.

 

[79INA69]

Just wanted to make sure you have the correct rotation water pump on there as I'm sure you know the late model 302's run a reverse rotation for serpentine belt set ups.

 

[DirtDonk]

...I honestly think its either fuel starved, an electrical issue (distributor and plugs look okay, but burned up 3 coils past week or so) or some weird vacuum issue.

Say what!?? How are you destroying coils, and how are you verifying this?
Have you checked your TFI module on the distributor?

Timing seems to be correct.

Are you just checking at idle, or are you watching it progress through the rpm range? You need to follow it along to high rpm, to make sure that the computer and distributor are able to raise the advance.
A retarded ignition is often a hot running engine.

What type of temp gauge are you using, and are you verifying the gauge is correct by using a thermometer to check it?
Is there a way to see what the computer sees from it's dedicated coolant temp sensor? OR do you just have to test it with an ohm meter? Wondering if the sensor is telling the computer that the engine is cold, and therefore it's running overly rich.

Replaced MSD Ignition and coil plus MAF Sensor and Fuel Pump, today which also made it run a tiny bit better.

If you have an MSD control module (MSD 6a?) then there's no excuse for burning out coils because the coil never sees long-duration voltage on the positive side and does not heat up the same way an old ignition would have.
Though I admit I don't remember how they're connected to a Ford EFI setup.
Are you still running through a new MSD box? Or did you eliminate that and are just running through the Ford ECU?

It does do that weird POP when its cold, think its probably because intake tube is too short and not enough space between MAF and Throttle body.

What weird pop?

It is something I've been meaning to replace, but has been that way for a while and wasn't overheating. Could also be a vacuum issue (the fox body 5.0's are notorious for), but I don't even know where to start with that.

Start with checking for vacuum leaks the old fashioned way. Spray some sort of fluid all around the engine's upper half and any vacuum hoses that are laying around.
EFI engines like this don't even react to small vacuum leaks, because the computer can compensate usually. But eventually a big enough leak develops and you get some funky running to let you know. But I don't remember hearing about running hot as being one of the symptoms.

However, spraying some junk around the engine while it's running will tell the tale. Even if you use water (I use Chemtool) any time you spray near a vacuum leak the idle speed/quality will change. If you detect a change, you've found a vacuum leak.
Brake boosters, unused capped off ports, hoses, gaskets, all of the above.
But if you have unused ports under the intake, replace those no matter what. Most non-factory caps are junk and last only a few months to a few years. They're an automatic suspect in any vacuum leak hunt.

Good luck.

Paul

 

[DirtDonk]

Just wanted to make sure you have the correct rotation water pump on there as I'm sure you know the late model 302's run a reverse rotation for serpentine belt set ups.

Good point. But I think he said he's got Explorer front stuff, which means he an only fit an Explorer (and maybe that one Mustang) pump anyway.
But it's still worth checking.

My first thought was the electric fan, but the fact that it cools down when slowing down, seems to indicate the electric fan is doing just fine.
But the Explorer fan would probably have been better overall. But the electric fan (especially a big one) can do wonders for low-speed cooling.

Paul