Negative results with the starting fluid.
Since the problem definitely changed/worsened with the disabling of the map sensor (even after re-enabling the sensor, then replacing it), does the “default” non-MAP computer setting do something that might have exacerbated the problem?
Do you have a tps like this:
You might try replacing it.
Yes I one of those. Is it a common failure?
I don’t know, but it would explain why the problem goes away at larger throttle openings.
I used contact cleaner on the TPS, the sensor next to it, and the ECU. Slight changes in the behavior, but nothing useful. So then I put my old MAP sensor in, and the idle smoothed out. Maybe it just got to the right temperature, but it appears much better.
Runs slightly better than the original problem, but still bucking.
Okay. I’ve been driving the truck a little more over the last couple of days and the original symptons have returned with one bit more information.
It runs fine when ice cold, then after about five minutes, the bucking starts. If I immediately cycle the ignition switch (less than a second), it smooths out. It will do this several times until 20 minutes or so has passed, and it then works from then on. The temperature gauge is showing the engine is the same temperature for all the failures and success.
My new theory if that is the ECU temperature that’s the issue. Ice cold works, warming up it needs occasional re-boot, then when warm, it’s fine.
I’ll remove the ECU and look for dirt, cold solder joints, etc., but if I don’t see anything, the question is If I replace it with the same p/n, will a new ECU work or do I have to program it somehow?
Have you tried replacing the coolant temp sensor for the ECU?
Tester
Why would re-booting change the result? I can shotgun all the sensors, but it seems like poor plan. Where is the coolant sensor?
Cancel post
I finally got my OBD-1 tester and here’s what I got:
Thinking I hadn’t pumped the accelerator properly, I started over and got:
I can’t find the Ignition Control Module to clean the connection, nor do I know where the Knock sensor might reside.
The Ignition control module will either be mounted on the distributor, or on the firewall, or near the radiator core support under a rubber cover.
https://www.rockauto.com/en/moreinfo.php?pk=4587906&cc=1121904&jsn=454
http://classicbroncos.com/tech/wp-content/uploads/2010/0/Fig_5D1-500x375.jpg
But, when there’s a problem with ICM, the engine won’t start/run.
Tester
I read that the SPOUT failure could be a loose ground connection between the ICM and Spout. Your photo shows me at least how large an object to search for. I’ve got a DIY manual in the mail. so my IQ should go up shortly.
SPOUT == spark output signal from ECM to ignition module. That’s the signal wire the engine computer uses to tell the ICM to fire a spark plug, so if that’s not working, you can see why the engine isn’t running well. See if you can trace that wire path, you may find a loose, broken, corroded connector. Or a wire who’s insulation has worn thin and is shorting out. The MAP sensor involvement is hard to explain, other than when the MAP sensor is disabled the engine computer uses a much simpler algorithm to set the fuel mixture, and it probably sets it richer than normal, which might mask an ignition system problem. In other words when the MAP sensor is plugged in the engine runs leaners , which is normal, but a leaner mixture is more sensitive to ignition system problems causes by a problem with the spark output signal.
My ICM is on the distributor (I think). The wiring goes from there to the ECU, right? Given the fact my truck runs fine for awhile if I cycle the ignition tells me (as a engineer that’s solved lots of problems by rebooting a computer) that it’s more likely the actual ECU. Unless it defaults at reboot to a version that runs well, then reverts to using the external data sources, which if not warm, misleads it into crappy running. I’ll check the wiring tomorrow and maybe visually inspect the ECU - my first removal attempt didn’t succeed as it wouldn’t budge from it’s nest.
Heat in the engine compartment can affect the connections in a broken or corroded connector too. Under good lighting very carefully, you may need to use a magnifying glass, examine the wire bundle from the ECM to the ignition module for problems. It may be necessary to separate the wires in the bundle into their individual strands.
p.s. if you’re not particularly skilled in electronics, probably best to farm this job out to someone who is. You want to avoid replacing the ecm if at all possible, as doing that replacement may introduce other problems, so you need to be sure that signal from the ecm to the icm is actually getting there in good shape before considering to replace the ecm.
I’m a hardware engineer, so wiring is duck soup.
Get out the wiring diagrams and your DVM, best of luck!! 
The computer (ECM) has a tag on it that says “Remanufactured” and “CC 06155175 P”. No trace of a conventional number like the ones expected or manufacturer. The dealer says the VIN database runs out before about 1999. If I search for my model on eBay (for example) I get many hits, each with a 12A650 in the middle, but different suffixes and prefixes. Any idea how important the other numbers are? Anyone have a 1990 XLT Lariat with a 4,9L?
Is whether it’s a Federal or California vehicle in the VIN somewhere? We live on the CA/OR border, so it could either.
For that information, I suggest you pop the hood
Somewhere on the underhood emissions sticker, it will answer your question. If it is a California emissions vehicle, the sticker will mention it. If it doesn’t mention it, then you’ve got a federal vehicle
So you’ve already got a remanufactured ECM. I certainly hope that the cause of your problem.
Here’s why I’m saying that . . .
At work a few months ago, I was working on a 1998 Ford F-150, which had problems with the electronically controlled automatic transmission.
The underlying cause of the problem was the ecm. It was not providing proper ground for a solenoid
Replacing the module with a remanufactured unit took care of the problem, and the transmission resumed normal operation
However . . .
A few weeks later, the vehicle intermittently wouldn’t start
This was a new problem, one that didn’t exist before the remanned module was installed
Diagnosis confirmed the remanned module was the reason for the no start.
Replacing the module yet again got the truck to a state, where it started reliably
So by replacing the original module with a remanned module, we essentially traded one problem for another
I strongly suspect that whoever rebuilt the module fixed the original problem, without “going over” the whole thing
I’m mentioning this because every time we swapped modules, we wound up with somebody else’s module, so to speak. I’m sure OUR module got rebuilt, but who knows who got it?
I believe the module is partially inside the engine bay, and partially inside the cab. You have to release a locking tab from inside, if you want to remove it
Intake Air Temperature sensor may be y our problem. On a cold start the engine runs in open loop until the coolant temperature rises to the normal range then the computer goes to closed loop and attempts to bring the O2 level under control but can’t if the air temp sensor is faulty.
And many Fords left the assembly line with computers that were remanufactured.