I have a 1991 f150 5.0.
my issue is that when at a stop the idle will surge high and low and sometimes stall the engine.
I thought that the issue was the IAC and it seemed that it fixed the issue at the time but after I fixed a stuck open thermostat the surging idle and stalling started happening again when the truck is at operating temp. Also after putting gas into the truck it will sometimes make a buzzing sound and not crank at all, this is usually handled by pushing on the gas while starting, idk if this is related or not. I do not have a obd1 scanner so I cant pull a code if it has one. my suspicion is that it is possibly a purge valve gone bad? but I don't want to replace the 50 dollar part if it does not need one.
Any thought/advice?

 

I had to drive it out of town for about an hour. First ten mins it ran good. Got to temp and started doing it again. About 30 mins in at 55 it stopped doing it all together and ran pretty smooth?

 

Hey Fetter,
I had the same issue on my ‘95. Just above your thermostat housing, you should have a Coolant Temperature Sensor. If it is bad, it will send a false temperature reading to your computer, and it will cause surging idle, and a few other issues. It won’t throw a code, because the computer doesn’t know the sensor is bad. It’s just reacting to whatever the sensor tells it. When the sensor is bad, the computer isn’t able to give the engine the right fuel to air mixture, because of the incorrect reading. Basically, the computer senses outside temperature via the Air Intake Temperature first, because the engine is cold. Once the truck starts, the computer runs off of the Coolant Temp Sensor. You probably see the same issue after driving, then starting it again. That’s because the engine is already warm, so the computer will default to the Coolant Sensor again. This can sometime make it difficult to start after the engine is already warm.

Trust me on this. I had the same issue, and thought no way it could be such a simple part, that I thought had nothing to do with idle. But, alas, that was the issue. There is an additional temp sensor, on the top of your intake. Don’t worry about that one. That’s the one for your temperature gauge.

Being that you had a stuck thermostat, you may have had some debris in the line (corrosion) and it faulted that sensor. It’s a very easy fix. Swap it when it’s cold, and have some rags ready. Water/coolant will leak out of where the sensor screws in. I don’t remember the socket size, but you’ll need a deep well. Don’t skimp on this part. Buy a quality replacement. Best of luck. Let me know how it goes.

You can see it’s the copper fitting above the thermostat housing.

 

If thats the case that might be the issue. I already replaced the sender thermostat because it was shot. I will replace it and let you know!! It looks very corroded too.

 

Yea, it does look corroded. I would take off that upper fitting too and make sure it’s clean too. Maybe check those small rubber lines for trash too. It can’t hurt. I changed that same sensor twice before it was fixed. Cheap Autozone and Oreily parts are really hit or miss.

 

Could the bad sensor be working some of the time? I drove the truck back from work and it had 0 issues even at temp.
I got the sensor and some tailgate hinges thatll br here tomrrow.

 

I dont think it was the sensor. I replaced it and it makes warms start a whole lot easier and runs better but I'll still get surging idles. The surging idle will sometimes go away if I floor it though. I fixed mostly everything so the it drives pretty smooth but the surging idle/stalling at stops is the only thing now. Any thoughts???

 

fuel pump, fuel pressure, in line fuel filter...check the fuel system

 

I pulled some codes using the check engine light method and I got 53, 95, 39. code 39 is from the ETC and made it so the truck didn't crank after warmed up. What does a code 95 in tale exactly? Also the 53 code is for the tps above max volt of 4.5 volts? Would the TPS cause the surging idle?

 

Not sure about the code meanings. I always google them, and someone has input and experience with them.

TPS can absolutely cause surging idle. I didn’t want to suggest that, unless you knew for sure. If you replace your TPS, make sure you get a Motorcraft. Do not settle for the parts store sensor, or you’ll be changing it again in 3 months.