I'll try to make this as short as possible. Just did some work on the ol 5.0 and now have poor running conditions and code 41 (HEGO not switching, always lean). The oxygen sensor was replaced about six months ago when I replaced both exhaust manifolds (cracked). Now for the work already done. I just replaced the injectors with a set of rebuilt stock ones. While I had access, I replaced the fuel pressure regulator, as well as the EGR tube that runs from the intake manifold to the EGR valve (old one was literally broken), and the check valve in the AIR crossover tube behind the heads. The truck starts and runs great when it is cold, but as soon as it warms up the throttle response is poor until you get to about 1/2 throttle, then it takes off like a scalded ape. The idle speed also fluctuates, like it is running out of fuel, then revs to about 1200 rpm, then calms down and does it all over again. I have not had a chance to check the fuel pressure yet, but I don't suspect a problem with it. Could the TFI be starting to go bad? I'm just trying to get an idea of where to start without blindly replacing parts. Thanks for the help.

 

vacuum line might be disconnected or loose. Maybe you bumped it while you were working on it. Had the same symptoms on my truck and thats what it was.

Hope this helps.

 

My dad's 87 F150 had a similar problem.We had replaced the driver's side valve cover gasket,and to get to it,the EFI plenum had to be removed,so we had to unplug alot of the hoses.We put it all back together,but it ran like crap.We got to looking all over,but we were stumped.Then I noticed this thing on the firewall had a missing hose.I dug around for a hose and when I found it,I plugged it in,and almost instantly the truck started to idle normal again.We felt so stupid.lol.

 

If it were a vacuum issue wouldn't it run crappy all the time, not just when its hot? It runs great when you first start it up. Only after it gets to operating temp does it start to act up.

 

I'm not sure,but I know that when my dad would rev the truck to high rpm's,it would mellow out.But then when he let off,it would have a choppy idle,and would sometimes die.It also wouldn't make it out of the driveway under it's own power.But I think different vaccum hoses could provide different results.Did the truck run fine before you did the work?If so,I would go over the entire vaccum system and make sure everything is hooked up.

 

The truck always had a little hesitation off idle, but not as bad as now. I'll double check all the vacuum lines again. Does anyone know anything about the TFI module? I have heard that when they go bad, the truck will run ok cold and bad hot. Is there any way to test them myself?

 

i have 86 302 EFI. I had a problem with random high idle up and down when truck got hot.
Turned out it was the Throttle Position Sensor going bad when it got hot.

 

I hope not, I replaced the TPS not too long ago. I will test it again, though. I probably won't be able to work on it again until this weekend, and I'll let everyone know what I find.

 

Check the EGR valve. Pull the vacuum line off and cap it when the engine is warm. If it's solenoid (the only round one on the drivers side valve cover) is sticking then it will remain open causing a hesitation and poor idle.

 

I've heard that the EGR can cause similar problems. There are no solenoid or anything like that on the drivers side valve cover on my '86, though. There are a few relays and things like that right behind the battery underneith the vacuum canister. Is that what you're referring to?

 

These are vacuum solenoids. They have a vacuum line going in, and one going out. There should be three. One for the AIR bypass, one for the AIR diverter valve, and one for hte EGR valve. The EGR valve solenoid has a large round vent on the top of it.

 

Just to update everyone, I thought I had found a vacuum leak where the lower intake maifold bolts to the head, so I tore it all apart again. Along the way I managed to break off one of the manifold bolts . Finally got it out with the help of my propane wrench. I was pleased to find that the engine valley and intake ports were very clean. I cleaned everything up and put it all back together. I also went over all of the vacuum lines and found a couple that were leaking. So now I'm feeling good, thinking this thing is going to run great. Well, I was wrong, it still has the same poor idle and missing at partial throttle. I checked the TPS, IAC, EGR and everything else associated with idle control and everything checked out good. I did vacuum tests on the AIR injection system and everything is good there. The only two codes I am getting are 25 (knock not sensed during dynamic test) and 41 (no HEGO switching detected always lean). The knock code has been there for a while, but I don't think it would affect the running of the truck too bad. I checked the voltage output of the O2 sensor (it is supposed to be between .35-.55 volts at an idle. Mine was jumping all over the scale from .8 down to -.5 volts. I don't know how it can read negative voltage but thats what the meter said. I'm going to replace the O2 sensor tomorrow to see if that fixes it. Is there anything else I should be looking for? I have been over this truck so many times, I'm starting to see double. If anyone has any other suggestions, let me know. I'm open to anything at this point.

 

The O2 sensor can do that.

 

Well, I replaced the O2 sensor today and the truck runs great now. I cleared the codes and ran the test again. This time I got 11 on the KOER test. I was happy to see that. Guess there is something to that computer thing after all.