Hi y'all!
Have a '98 F-150 XLT, 4.6L, 2WD with nearly 250K miles. I guess I should be thankful it's lasted this long but really would like to figure out why it overheats intermittently.

Started with bypassing the heater core, then the thermostat, then new water pump & radiator (& flush), then (not related) the alternator. Worked great for about two weeks then started overheating again...not gradually but all at once red pegged! Suspected faulty thermostat so replaced that & flushed again (it had quite a bit of rust in the water before so thought it couldn't hurt). Worked great again for two days this time. At this point, I'm thinking head gasket(s) but here's where the scratching comes in.

I drove it for 3 or 4 miles from a cold start this morning & it pegged the needle, so I stop & shut it off, restart after a couple of minutes, shows normal temp for another couple of miles, then pegs the needle again. Stop, shut off, wait & restart. This time, drove it for about 8-10 miles & it never came off normal temp. This has happened more than once & it's driving me crazy! There are no external fluid leaks, hoses are good & coolant topped up. The only time coolant level was low was after it boiled out the reservoir cap (not on tight enough) but that was a minimal amount.

I am wondering if it could be the fan clutch.? Even thought it may have jumped time because of the way I can do the stops & starts & it not overheat then. I'm grasping at straws & could really use some additional thoughts. Thanks in advance & sorry for the length.

Wayne

 

You have not established that the motor is actually overheating.
My bet is the gauge is fooling you with an electrical fault by acting so fast then appearing to clear intermittantly.
Here is the analysis:
Since your not indicating the PCM is going into limp mode from the CHT sense line, I think you have a dash issue.
The input signal to the dash are CHT, Oil pressure switch, and vehichle speed sensor.
The outputs to the gauges are the Tach, oil pressure indicator and coolant gauge.
If only the coolant gauge is acting up, it seem to narrow it to that function.
Some others here may be able to add some additional help on the dash issues.
Good luck.

 

If only that were true, I'd be a happy camper. When it red lines, the engine sputters with very little throttle response. Wouldn't do that if it were a gauge issue, would it?

 

If that is the case I believe the PCM is being signaled off the CHT line to go into limp mode falsely from a electrical signal condition.
The limp mode causes cylinder skipping to help limit the engine temperature for a short time by cutting off the injectors on the cylinder to be shut down each time the cycle runs.
..
That puts the problem on the sensor input side of the dash rather than the output side where the gauge is operated accordingly.
What needs to be done now is use a good scanner that can look at the CHT line to see what is happening.
This is the new computer control now.
You have to go beyond to see what is happening rather than guess and replace parts on a hope.
This is why a Scanner is used on the DLC port under the dash for more than just diagnostic codes..
Seems like this same problem was just recently visited some other place.
Good luck.

 

May sound stupid, but do you have air in the cooling system? Ive seen air get trapped in the system and it will "read" hot, but a temp gun never shows out of spec readings except for around the thermostat housing or CTS.

 

Yes, if you have air in the system, the air will accumulate at the top of the water jacket, where the water temp sensor is. I have seen several times , the gauge spike within 20 seconds, then drop back to normal when the water hits the sensor. You need to determine if the air is because you have not filled it properly, or have a head gasket leak. I had a small leak. NO problem when running. Park overnight and the 15 psi in the radiator would push an ounce of water into one cylinder. Startup in the morning was sometimes a little rough, would cleanup in 30 seconds. AFter 3-4 days of this the water level would be low enough to get the "air pocket" in the motor by the sensor and get the overheat signal. Monitor the water level daily and mark it. A drop of 1/2 or 1 inch is suggnificant over a few days.... I fixed my problem with BARS copper head gasket sealer that you mix with antifreeze. Dont have to drain radiator. Been 2 years now.

 

If a head gasket leak is bad enough, it will pressurize the overflow tank and make the cap "pop" and spray several ounces of antifreeze all over. Now you can have an air pocket due to the low level. Hard to know if the cap popped first and then overheat, or the overheating occured and then pop..... monitor your level.

 

I would properly bleed the cooling system. Start it up from cold, then top off with coolant mixture and let warm up with the rad cap off. Wearing safety glasses, look down into the reservoir (with engine running) and watch the coolant level while squeezing the top radiator hose with one hand. Watch the fluid level in the res. tank. If big bubbles come up, there is air in the system. Keep doing this procedure for about 1/2 and hour while the engines warms up with the pressure cap off and while continually squeezing the upper rad hose. This will purge all the air out of the cooling system (if that's all that's wrong, air in the system) Once the fluid is clear of bubbles after about 30 to 45 mins. You can then be reasonably sure the air is all out.

Then drive and check every day for a while and see if the coolant level drops further, some residual air may purge out. Once the level stops dropping, the air has been purged. The air pockets tend to get stuck in the heater core and move around the engine making the temp gage spike and then suddenly drop while the air is circulating. It can take quite a while to manually purge all the air out as described above.

If white smoke is detected coming out of the tail pipe, you've most likely have a blown head gasket issue. Also check the engine oil for signs of coolant, head gasket issue again.

 

Sorry it took almost a year to let y'all know the outcome but, you know.

As it turned out, it was a combination of things including, hole in radiator at the top weld, bad water pump (impeller looked like a throwing star, new one fixed it for a couple of days) & finally, blown head gasket. Finally quit running hot & has not since, BUT, about a month after it was "fixed", it started missing & backfiring through what felt like (in the gas pedal) the throttle body. It would straighten out at acceleration in higher speeds & after it warmed up. A couple of weeks later, it stopped doing that entirely, so I thought the problem "worked itself out". Yeah, right!

It ran great up to about a month ago & now it misses from start up & continuously & backfires on acceleration until I reach a cruising speed (65 or 70). I'm suspecting an exhaust leak or maybe the firing order is wrong since he did change the plugs when he did the head gasket. I tried searching for the order here from previous posts but turned up empty. Does anyone happen to know that info? My owners manual has mysteriously disappeared. Thanks guys for all the input before!