I got this truck from my neighbor because he thought the transmission was going out. It felt like super nasty torque converter slip under load (took it to a trans shop, they agreed). After I put a new transmission in it, the problem was still there. Finally figured out it was a nasty misfire, slight at idle, very violent under load, shakes the whole truck. When it downshifts and the RPMs pick back up, it goes away.

I have replaced the coolant temp sensor, TPS, pick-up coil in the distributor, O2 sensor, plugs (gapped to .054"), wires, cap, and rotor. I have restricted the EGR valve down to about 1/4" and verified the timing at 10 degrees. When I put the new MAP in (the last thing I've replaced) the problem got way worse.

My next move is to replace the ignition module, it's about the only thing left besides the EEC. Has anyone experienced this problem? I just want to get the stupid thing running correctly so I can sell it. Any help is greatly appreciated.

 

Did you pull the codes before all of these parts were replaced? Is your vac line to the MAP sensor in good shape?

 

There are no codes. It's OBDI, it barely knows it's running. I got a misfire code once and it was barely running as I was limping it back to my house. Absolutely everything vacuum related has been replaced inspected. I have checked for leaks everywhere, there are none. I've had the intake off, replaced all those gaskets and cleaned the hell out of it.

 

We know it's OBD-I based on the model year you posted.

I disagree "it barely knows it's running". The EEC-IV powertrain control system does have some intelligence and is quite reliable.

There is no misfire code for this vintage truck. Erratic PIP, yes......no for misfire.

 

What condition is the smog(air injection) system in? A leak in this system up behind the motor will cause this extreme lean stumble you are experiencing.

 

Hell, just pull the PCM and take a look inside of it.

If your just throwing parts at it and don't have codes, its pointless. I would, if I had this much time and money in it, see what the PCM looks like inside. Your looking for bulging, leaking, blown, capacitors. If you SEE ANY green corrosion around the base of the legs going into the board, that's a RED flag and also if you see any residual fluid under any of them.

 

That's my next move. It sat for two years outside.

 

I just pulled the computer out, it looks just fine on the inside. It is a reman, and I can't find the part number anywhere on the Internet. The factory tag is painted over, so I don't know what the code was. The part number on it is 103218523612757 REMFG.

 

I also removed the entire vacuum harness and tested all of it, no leaks at all. I pressure tested all the valves and solenoids, none of them leak. The hose clamp for the hose that feeds the crossover tube for the back of the heads was loose. After tightening that the problem has gotten better, but is still there. The only thing left to replace is the ignition module on the fender. Any thoughts or insight?

 

I do not believe we ever got a clarification on the code question. "No Codes" as you previously replied can be interpreted many ways. I read that as you never got the System Pass code of 11 or 111. Please clarify.

There is no way to look up a replacement PCM with the numbers you listed. You can post the Calibration Code sticker located on the driver side b-pillar. Subford or someone else could look up what PCM is needed. Since Motorcraft revamped their website it is very difficult to lookup yourself these days.

I assume you also checked the vacuum reservoirs for leakage?

Since the problem got worse with a new MAP did you put the old one back in or test either?

Fuel pressure been tested at idle and under load?

 

The last time I checked for codes I got the 11 or 111 pass code. The check engine light is currently not on and has only gone on once. The misfire was way worse with the new MAP sensor, but the transmission finally shifted the way it should. After I tightened that hose clamp I mentioned, the misfire was back to the way it was or a little better, leading me to believe there was a problem with the MAP sensor. When I pulled the vacuum lines off of the reservoir, it started sucking air, so it is good. The smog pump works, the intake hose is sucking air. The fuel pressure regulator is new, I replaced it a couple of years ago for my neighbor, it was running 80 PSI at an idle. I checked the pressure the other day, it's within spec at idle and increases with throttle input.

So from all of my testing and inspection, the only thing left is the ignition module. Unless it has the wrong computer in it, which I find hard to believe because the truck ran right at one point.

 

So what were the measured pressures? Yet again you only give partial answers. No one here can help you without having empirical values.

Just because you replaced the MAP sensor does not mean it is good. What is the output frequency at idle?

There is more than one vacuum reservoir, which one did you test?

 

IIRC this model has metal tubes behind the motor that bolt onto the heads for the smog b.s. if one of those tubes are broke, rusted through and had holes in them it'll create this problem

 

Fuel pressure is 33 at idle, 38 with the vacuum line pinched off, within spec. I'm not trying to give "impartial answers", if you need numbers down to the tenth decimal, I'll try to get those for you. There is only one vacuum reservoir under the hood, on the right fender. There is a charcoal canister, on the right frame rail, but I haven't tested that. Which wires do I probe to measure the MAP frequency?

The truck only has 99,000 miles on it, and zero rust or corrosion of any sort. I guess the next step is to actually pull off the head crossover tube and all that other smog crap and check it out.
I'm also assuming that if I remove all the smog stuff, it'll have a check engine light?

 

There is another small vacuum reservoir for the HVAC system located on the side of the blower motor case. It typically shares the same vacuum line source as the MAP sensor.

courtesy of subford

The reason for asking about values is a very frequent misinterpretation of "in spec." Many times that has lead the OP down a bunny hole. I am not trying to be a troll and ask for something without reason.

For reference: MAP Sensor

The charcoal canister does not hold a vacuum, it's part of the venting system for the fuel tank(s). You could check to see if the CANP solenoid is fully closing. Even if it were slightly open it should have no effect on the engine, the feed into the engine is before the throttle plates.

You can remove the TAB/TAD hoses and associated air pump plumbing. Leave the TAB/TAD solenoids electrically connected to keep the Check Engine Light off. You will fail a KOER test but that will not trigger a CEL.