KurtH

Hello,

My F-150 just will not run well. Very suddenly, after starting up, the idle goes, up, down, then down so far it stumbles, and eventually finds a an Idle speed around 1000. When I step on the clutch, the idle surges, and it might even stall, then it might or might not turn on the CEL. I cleaned the IAC, and replaced two of the soft vacuum lines on the top which were really cracked and nasty. .. no change. Some research I did at that point suggested it was the EGR valve, and it is so rusty I know I would never get it off.

I had it towed to a shop I have used before, and they replaced the MAP sensor, which helped a little, but it still stalled and ran weird, so They also ended up replacing the distributor, and ignition module. I drove it home (5 miles) and it ran smoothly, better than I remember it running in the two years I have owned it.

Any way I went to drive it 3 hours later and the symptoms all returned. I parked it for two days and then checked the codes (i wish I had done this before towing it in) and it indicated two codes for the ECT sensor. The wiring looked ok, so I decided to replace the part, and it helped a little bit, but the truck is still un-drivable. Out of frustration I unplugged the Throttle position sensor and started the truck up and it ran about the same as it did with it plugged in. Bad TPS? bad wiring? I could not get it to turn on the CEL again.

Any other ideas? Thanks in advance for any advice you may have. I have been searching on several message boards and every resource I can locate, and and can not find anything quite on point.

memoniz

It sounds like a TPS to me. Don't have much experience with it, but a friend had a problem with his truck. It would start and idle fine, and you could drive a short distance but after that it would become erratic and buck and run like crap until we got it back to his garage. We tried Seafoam in the vacuum lines, ran a little better, but not much. When I left, I text him to look into the TPS and read the symptoms. He eventually replaced it and has not had the problem since. You might want to try to clean the throttle body first. I would read the posts on the TPS before I bought one to see if any of their stories sound familiar.

KurtH

I pulled off the negative battery cable, let it sit, then re-attached it started it up, it then let it run a bit, it stalled and threw more codes, I used the "count the blinks" with the engine running and got 213 and 116 SPOUT circuit and the ECT again. The procedure int he haynes manual suggests ignition module problems, which may be why the distributor and ignition module got replaced. I will run through the test procedure when it is not so hot outside. This procdurewhich also leads to the computer, and somewhere on this forum I saw a post suggesting wiring problems which led to a recall back in the day. So maybe after 23 years I have a short somewhere.

I am trying to figure out why it ran perfectly for the trip home, the only things I can figure is the wires got moved all around while it was being worked on, and then settled down to the normal resting place. Or if the PCM is indeed bad, having the battery disconnected while the shop was working on it drained all power from the capacitors in the PCM, masking the problem.

And the adventure continues....

Crock Pot

You might try replacing the coolant temp sensor. If it is not working and telling the ECU it is cold when it isn't, then it will turn the choke on, or give a rich mixture. Easy access for replacement, not too much $$.

KurtH

Since my last update, I did in fact change the Coolant temperature sensor, using the paperclip method, it reported two codes for the ECT. I then had to drive around town to find a deep 25mm socket. I ended up with an impact socket. The code went away but it did not help.

I got ahold of the Inova code reader, backprobe leads and a vacuum pump. I got codes again, and they indicated the Throttle position sensor, as well as EGR issues.

I used the test procedure in the Haynes manual, and when applying vacuum to the EGR with the engine running it stumbled, which the manual said was expected.

I moved on to the TPS testing, using my backprobe leads, and following the Haynes manual, the readings were no where close to what the manual indicated. I went to get a new TPS, but I had to wait until the next day to get the part, it was in stock at a store that was just a bit too far away to drive to.

So that brings me to today, I replaced the throttle position sensor and there is marked improvement. I took it for about a 10 mile test drive, and it is much better, but still feels a little strange. I will try to describe it.

first, at a stop light, the idle goes up and down intermittently, like it wants to stumble but catches it self

second, which is a bit harder to describe, if I accelerate, then take my foot off the gas, the throttle response feels more like a switch, than a smooth transition.

also of note, the IAC connector on the harness is a little dodgy, i found the wires wrapped with electrical tape, I took all the tape when troubleshooting this before sending the truck to the shop, and found one wire soldered right at the plastic, and the other wire was down to just a few strands of copper,and the insulation cut away, as though someone tried to probe it with a normal test lead. I cut and re-soldered this wire, i had enough slack to cut it back to good wire.

I did clean the IAC earlier but I wonder if it might be worth replacing it, as well as the gasket, and the connector.

I also stumbled across a youtube video of someone replacing the throttle body gasket, apparently at some point there was a recall back in '96 for the throttle body gasket.

It is not throwing codes any more but like I said, it is still just not quite right, but closer.

Any other thoughts are greatly appreciated.

redleg122

Id Like to maybe make a suggestion, i had a similar issue. So i removed the "Coffee Can" and stuck a bolt in both vacuum lines. Hope this helps.

memoniz

From what I understand the TPS is pretty touchy. It needs to be set right. Read some other posts about it they give you a clue. Also I would run the tests on the TPS again to see if the new one is falling in range before I move on to something else.

KurtH

Thanks. After letting the truck sit for an hour, all the symptoms came back. This is exactly what happened after bringing it back from the shop. Further digging into this forum, there were some discussions about checking fuel pressure, and how the fuel rail can get warm causing vapor lock.

About the coffee can ..
I found two coffee cans .. one on the passenger side towards the front of the engine compartment, and another under the overflow/washer reservoir. It looks like the one under the overflow bottle is dedicated to the brake booster.

I will have a go with the Throttle position sensor again, I will try adjusting it.

The weird thing is, that this truck was running great, It was my daily driver and even seemed to be getting improved fuel economy, then I parked it for a long weekend when I went out of town and all of a sudden... bam .. all this started.

memoniz

Yea, weird things happen on old trucks. I had a starter relay go bad on me the day I changed my distributor. And the relay was maybe a year old. That's why I retest now, to make sure what I did is working and not assuming what I did was right.

Crock Pot

The up & down idle sounds like an air leak in the intake somewhere: gasket or vacuum line.
When you park it after the test, is the hood closed? I popped mine open to let the hot air out so if there is vapor locking in the fuel rail, maybe it will cool off before you try to crank it again.
How full is the fuel tank?
Are you running 100% pure gas or E-10? E-10 will cause problems if it sits in the tank more than a couple of weeks. Humid summer air will provide enough moisture for separation.
I have all kinds of as yet undetermined issues when the tank gets down to 1/2 full. Fuel pumps are new, as are most of the sensors. Either of the pumps will run and run with key on engine off, and not get the system pressurized. It will empty a full tank on a long run with no issues, it is just restarting after an overnight. Once started, it is about perfect. It's just not getting fuel from a cold start on less than 1/2 tank (either one).