Help! High Afr Warm Start


New Member
Sep 7, 2016
Hey everyone,

I've been a long time reader here, and have never run into a problem I couldn't get an answer to by just using the search bar.....until now.

First, some pertinent background info about my car:
1965 Mustang Coupe
'89 302 HO / T5
E303, GT40's
Summit Intake manifold and 600 cfm holley style carb
Electronic fuel pump (carb specific)
Always runs 93 octane
Heater core bypassed/removed
3 Core aluminum champion radiator
Approx. 1.5L overflow tank
New water pump, thermostat (180 degree), hoses
17" mechanical fan with shroud, run directly - no fan clutch

My 65 mustang fires up immediately and runs great when the engine is cold, and I can drive it for as far as a tank of gas will take me - be it highway driving or stop and go traffic - without the original dash mounted temp gauge every breaking 1/3 of the way past C (reads 180 F on a stand alone mechanical gauge) as long as I don't make any stops. During this same initial operation period my AEM electronics AFR gauge (connected to a bosch 4.9 LSU O2 sensor) reads between 12.8 and 14.7, averaging in the mid 13s - right where I want to be. Unfortunately, if I make a stop at Lowes, the grocery store, etc when I come back out and try and start the car again it does not immediately fire up. Rather, I have to crank it for a noticeably longer time, and then when it does fire, it runs very poorly - fluctuating, low rpm's that force me to give it gas in order to prevent the car from dying. In addition to this behavior, and likely the cause of it, my AFR gauge will read in the high 17s and beyond, to the point that my AFR gauge only registers three red dashes (---) because the AFR is outside the bounds of what the O2 sensor detects. Since running the engine at a high AFR (lean) causes detonation, which in turn produces excessive heat, coupled with the fact that I am forced to crack the throttle in order to try and supply more fuel to the engine so that it wont die, thereby increasing the frequency of detonation is what I believe to be the cumulative cause of my car overheating after a warm start - not to mention running very poorly upon restarting. However, it is also important to note that if I get the car out onto an open stretch of road where I can open it up, then I can get the AFR to drop back down to just above stoich (14.7), but not quite back to my ideal range of mid-high 13s - granted by that point the temp gauge usually reads just under H and I can already smell the escaping coolant if I'm not already getting it sprayed all over my legs through the hole in the firewall.

***Note: Even at those high AFR readings, I do not hear the tell-tale 'pinging' of engine detonation.

Since every bit of my cooling system is new and the block had the water passages cleaned before assembly, I began looking into vapor-locking as the cause of my high AFR overheating issue. I rerouted my fuel line to the carb so that it had adequate clearance from the water pump and any other hot bits of the engine bay (its all steel braided fuel line back to the tank), I wrapped my electric fuel pump in reflective heat tape to ensure it wasnt getting heat soaked, and I wrapped my high-torque mini style starter in reflective heat tape as well for good measure even though the minis are already supposed to improve warm start conditions since it sits further from the header collector tube.

Unfortunately, now I am at a loss as to how to go about tackling this high AFR overheating situation. A friend suggested that I could replace my mechanical fan with a 2500+ cfm electric fan that I could rig up in such a manner that it would run any time the temp hit 180+, even if the car was shut off, so that it would reduce the potential for overheating when restarting the car. However, this seems to me like it would only fix the symptoms and not the true problem, the high air-to-fuel ratio, still leaving me with a hard-starting-when-warm condition. The only other thing I can think of would be to increase the size of the main jets in my carburetor so that more fuel is supplied to the engine, but then I would also be getting more fuel during the initial run period causing it to potentially be too rich when starting the car cold.

Let me know what y'all think, and thanks in advance for any advice or info!

PS (After being shut off for however long a standard stop takes, the mechanical water temp gauge reads ~200 F, and I believe that just over 210 is when my radiator cap gives way and allows coolant to spray all over my engine bay....and legs)
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New Member
Sep 7, 2016
Hey 65KModel,

I do have a catch/return tank attached to my radiator, its approximately 1.5L in capacity.


New Member
May 19, 2002
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You may need a return style fuel pressure regulator and a return to tank fuel line if you don't have one to go with your electric pump. Or you could try putting a mechanical fuel pump on the engine and removing/bypassing your electric pump as well and see if that helps.

Externally mounted fuel pumps get hot and fuel can begin vaporizing in the pump.

Modern cars have their pumps in the tank in addition to return lines.

A nonmetallic carb spacer may help some too.

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