Hey all you 300 guys out there. I need a little bit of help for my truck. I have an 88 F-150 4.9 with a 5 speed. It's got about 193,000 miles on it. It has developed a mid throttle miss (well, it also has the common small miss at idle, but I'll work on that after I get the other miss cleared up unless they're the same problem). The only way I can describe the miss/stumble is that it only occurs when you're cruising at part throttle. If you floor it, the thing takes off like it wants to snap the driveshaft. You wouldn't even know that there is anything wrong. And when you idle, it runs pretty decent. Like I said, there a small miss, but that idle miss has been there for the last 4 years that I've had the truck.

Let's see... Here's is a list of things I changed recently that may ot may not have something to do with what's going on.

-EGR Vacuum solenoid
-O2 sensor
-Spark plugs, wires, cap, rotor
-Kock sensor

I'm pretty sure the miss developed after the O2 sensor or the EGR Vacuum solenoid. I've been trying to figure this out for a few weeks now and it's really starting to get on my nerves.

As for codes the computer is reading:
KOEO
67- A/C on during KOEO or neutral drive switch open or manual level position sensor out of range or neutral pressure switch closed or Park/ Neutral switch open

Memory:
11- System pass

KOER
25- Knock sensor not detected during dynamic response KOER

Judging by the codes the computer is giving me, I'm going to say that the knock sensor I put in is no good. But would that cause it to stumble so badly? I think the code 67 is for reading the codes without holding the clutch down. Maybe someone can correct me if I'm wrong there. From what I've read and know, I'm definetly thinking that it may be a vacuum leak somewhere. I have tested with the old starting fuid test and I never heard the rpm's go up. Is that a sure fire way or is there another way that I could test for a vacuum leak? The last thing I thought of is the fuel pressure regulator. I pulled the vacuum line off of that and the idle miss sounded like it got a bit worse. Can anyone tell me how to test a fuel pressure regulator. Anyway, I'm kind of rambling on here. Any help you guys can give me would be greatly aprecciated.

 

Ok. I know there's a lot of reading to do to get down this far, but I have a new piece of info from further testing today. I disconnected a vacuum line and the idle went up. I left the vacuum line off and took it for a ride and the miss was gone. Any ideas? Thanks

 

The codes you got are probably because you didn't keep the clutch pushed in while you did the KOEO and you'll get the knock sensor code if you don't perform a brief WOT when the scan tool tells you to.
A common problem that causes a rough run at idle is the throttle body gasket being sucked in.
If the miss is at about 1300-1500 rpm then it's quite likely because you have to much EGR flow. You can test for that by plugging the vacuum hose to the EGR valve and then driving it. If it's better you can build a restrictor plate to go in between the EGR valve and the intake manifold. I think you'll find more info on that in here is do a search.

 

My 91 had the same problem, stumbling when throtttle was between accelerating and coasting...more so at mid speeds, not so much at 75. It died on the road yesterday, had it towed and the shop replaced the ignition module on the front of the distributor. Could not believe how smooth it was on the way home! I had tried to get the module off, but one of two screw stripped, would have had to pull the distributor. Did not know it was the module when it died, but just wish I could have gooten that replaced myself the first time! Good luck with yours.