I’m still chasing some electrical gremlins on my 93 F250 5-spd, 460 4WD.

I did a little bit of research into the EEC computer and the only thing I’ve found so far is that a computer on its way out will often keep the fuel pump running in the key-on/engine-off state.

Here’s my issue. At least once EVERY time I drive my truck the temp gauge will race towards the hot side of the gauge. When this happens the idle will come up, as the system is designed to. I really don’t believe an engine can heat up or cool down as quickly as my problem presents and then corrects. The truck will go from normal operating temperature to almost in the red, then back to normal over the course of a minute.

I took these pictures on my drive home tonight to illustrate.

Here I am, cruising along at a steady throttle setting and the temp gauge reads where it should.



I wasn’t fast enough to catch the climb upwards, so here’s about where it gets to before starting to come back down:



The next couple of pictures were taken over *MAYBE* a minutes time:





With an iron block and heads, I just don’t believe there’s any way possible that the engine could heat up so quickly OR cool down so quickly.

I changed from the stock 195 degree to thermostat to a 180 degree thermostat which has taken the problem from where the truck was nearly undriveable due to this issue to just a once or twice per drive thing.

When I bought the truck it had a gutted thermostat and I flushed and put new coolant in with the 195 degree ‘stat. I didn’t see anything during the flushing process to indicate a blockage or something mechanical, but I’m assuming the gutted T-stat was the PO’s way of combating this issue.

This weekend I plan to buy a mechanical temp gauge to run into the cab temporarily to see if it jives with what the gauge and injection is doing. I assume that it will, to a degree, but again, this thing simply cannot get that hot and cool down this quickly. Unless there’s a computer issue as well.

I checked the ECU coolant temp sensor, with help from a member of the forum. It checks out as I’m able to test it. I may also run a voltmeter into the cab to see what the readings are when the “weirdness” starts.

My ignition base timing is a couple degrees off… unfortunately it seems the distributor shaft has “welded” itself into the block and I’m pretty certain my best option will be to destroy the factory distributor getting it out. I’m prepared to do so, but want to make sure all my ducks are in a row first. I’m sure the 3-4 degrees of timing it’s off are just wear on the gears, but the computer should be able to compensate, to a degree.

And the ECU temp sensor reads within the prescribed values cold and warm. Every now and again the truck will get hot enough to turn the check engine light on momentarily, but I got all the codes cleared last time I was messing with the truck… the time I discovered I could not budge the distributor.


Here are my thoughts as far as electrical goes:

I’m very much starting to lean towards either bad computer… or possibly bad/old ECU wiring harness? I know it doesn’t take too much corrosion to really mess with the later CAN/BUS stuff, I suppose that *could* explain the high idle, but it comes in concert with the gauge sensor reading high as well, so I think it’s less likely. Unless the engine wiring is going bad… both sensors are based on measured resistance, so I guess they could both have a few more ohms that originally intended.

More I’m thinking the computer is starting to fail and isn’t “switching” as fast as it once did and maybe THATS the source of the rapid heating then cooling back down?!? Possibly coupled with the ignition timing not being spot on???

And the there are some purely mechanical thoughts:

Another PO “solution” was to remove the air-pump and I don’t like the belt arrangement he went with. As it sits, the alternator is driven by a significantly shorter belt, off the fan/water pump pulley. The crank pulley and the rest of the accessories are in stock configuration.

I’m thinking *maybe* not having the crank turn the water-pump/fan pulley with two belts has worn out the fan clutch?!? There’s also RTV on the water pump, a sure sign it’s been replaced once… maybe they put a counter-rotation pump on?!? I didn’t notice anything to indicate that when I flushed the cooling system, but it’s something else for me to consider. There’s no noise from the fan-clutch and the truck tends to present as “hot” going down the road, or sometimes right after exiting the interstate. Typically bad fan clutches lead to overheating issues at lower speeds… typically you HEAR them as well.

I’m putting together a rock-auto order with a new fan clutch, ac compressor clutch, distributor and some other ends I need. I don’t really want to buy a new computer, but they’re not SO expensive to not just do one, provided my diagnosis has any bearing in reality. At this point I really want to stop screwing with this issue. The new thermostat made the truck drivable again, possibly a 160 degree T-stat would solve all of my problems, but I would rather stick to what the engineers thought best and put the 195 degree T-stat back in.

As always, any assistance or advice the community can offer will be considered if nothing more. Thanks all!




Be safe


Jeremy

 

The temp gauge has it's own sender which does not go thru the ECU. The ECU gets it's temp signal from the ECT sensor. By your description, the temp sender and the ECT sensor agree the truck is hot, so perhaps it is. Putting in a mechanical temp gauge to check the actual temp is a good idea.

 

It’s definitely getting hot… how much so I can’t independently verify, but I’ve used an infrared thermometer as one of my diagnostic tools. Even with the gauge reading nearly in the red the thermometer doesn’t pick up temperatures anywhere in the cooling system that jive.

It’s hard to say what the precision of the factory gauge is, my guess is “C” is 100 F. and below and “H” is 230-240 F.

Infrared thermometer puts the radiator around 180-185 F. upper radiator hose in the same range and the only part that’s ever gotten a reading above 200 is the T-stat housing.

None of that is to say the two temp sensors, both new and measuring within spec AREN'T working correctly. Just to say that nothing else confirms the readings they’re giving. Neither my nose nor the thermometer agree with the what the temp gauge and ECU are seeing. I’m not puking coolant, the overflow tank level stays consistent, etc.

Additionally there’s just no world where a lump of cast iron actually heats up or cools down as quickly as this issue clears. Those realities have led me away from viewing this as a mechanical issue.

 

Incorrect timing can cause fun temperature swings as the combustion events get screwy. And FYI the computer assumes the base timing is 10 degrees btdc and advances it from there based on a program. The exception being some of the later models with knock sensors which will pull timing if knock is detected.

 

Also, if this overheat thing only happens once a drive cycle, it is probably just the heat build up before the thermostat opens, and then the crash after it opens.

The most common computer failure is leaking capacitors, which is usually apparent with a visual inspection of the board. The apparent failure symptoms are random according to which circuits are damaged.

 

That’s kind of my thought regarding ignition timing and why I made mention. I kind of have this feeling the truck is having a hard time getting into closed-loop and am thinking definitely get base timing set correctly to start and then look towards computer and wiring.

I also agree that once a drive cycle wouldn’t scare me to just see the temp spike and drop down. That would be just the T-stat cracking and doing it’s thing. Problem is I’m in Hotlanta… and we’re not even to the hot part this year. The 180 degree T-stat helped the problem become once a drive cycle… through the spring. But as the ambient temps rise I’m not sure that will hold. I watched the gauge spike twice today, which also leads me to that *something* far enough out of value that the truck is over-cooling, putting me into a rinse, wash, repeat cycle.

I went ahead and ordered a distributor, fan clutch and some other bits last night. Hopefully having a replacement distributor on-hand will mean I can free up the existing one. Surely it would break if I didn’t have a replacement.

I’ll start with getting base timing to 10 degrees BTDC, with the ignition “pill” removed and see where that gets me.

Knowing I can do a visual check of the ECU means it’s worth taking the time to have a look there.

I don’t know what it is about Ford trucks, but leaking capacitors seems to be a trend for them. I know the older variants had lots of trouble with the “brain-box”… once you saw goo on the inner fender you knew you were in for trouble.

 

EEC that doesn't record known faults is a dead giveaway. If you pull a sensor while running for example.

If you're crutching with a 180* t-stat, there's a problem somewhere. Even a 460 should hum right along with correct 195* unit in Atlanta in July.

EEC in these things were never expected to last this long. That they function with the efficiency they do with a certain amount of care is a tribute to their robustness & resiliency.

 

So you’re thinking EEC? I’ll pull the ECT sensor with the truck running (I’ve become quite familiar with it) and see if the check engine light comes on. That’s another place where I wasn’t sure of the difference between OBDI and OBDII. I know OBDII CEL stays on until codes are cleared and fault is corrected. The check engine light will illuminate, BRIEFLY, but only in the most egregious circumstances. From previous experience, it will store codes, I’ve dealt with the 34 (fixed) for the EGR positioner and have seen the 81(?) for ECT wiring fault.

Still the same advice, visual inspection of the EEC ECU prior to replacement?

Thanks everyone for helping me through this. After replacing almost every single injection sensor there’s only so many more things to look at. Unfortunately my cheap and reliable old truck isn’t turning out to be so cheap…



Be Safe



Jeremy

 

yes, pull EEC open up, and look inside, the idiot gauge inside truck(single wire temp sensor), has nothing to do with trucks issue, that could be pipe tape, or corrosion in threads causing erratic readings, but if your ECT(2 wire EEC sensor), sees too hot it will go into limp mode, sitting at redlight, all of a sudden high idle, your ECT sensor(bad when hot), or wiring issue, could be the cause, thorough testing, not the parts cannon, I think if you unplugged the idiot gauge you wouldnt even get a CEL, on , or stored, it doesnt go to EEC.

 

EEC-IV can record no-light, brief illumination & constant. Some codes are wiped in X# cycles. All are wiped with bat disconnect. Some people can wipe them pulling jumper during test.

I'm of the opinion (take with grain of salt) that acquiring correct duplicate ECU or near same year/equipment match & verifying operation should be SOP for long term ownership of this gen truck.

There are companies that can repair/rehab them & can generally be had for under 2 bills.

I also pick up used Motorcraft parts from the JY & test, then test on the engine. I test temp sensors with water & a candy thermometer & DVOM

 

And as hard as they are getting to find, stash house is good idea.

 

I’m not a shill for them… and wish they still gave the option to ship all the parts from one location with slower shipping, but I’ve found that rock-auto typically stocks OEM replacement parts. Plenty of Motorcraft options there. Not for a computer… but I’ve always heard that Fords in particular like Motorcraft parts and try to use them where/when I can. There’s a price premium, but I’m in this thing far enough that a few more bucks for what’s hopefully a better part seems to make sense.

Unfortunately there doesn’t seem to be an economical new/old EEC option. There’s a couple aftermarket companies selling rebuilt EEC’s or I can take a chance on another original computer. I do have a guy that has a particular penchant for the medium-duty OBS chassis… if he’ll cut me a deal AND has the part I’ll consider a “new” old ECU. At least as a diagnostic tool.



Be safe



Jeremy