Hi,

I have a '97 F-150 with the 4.6L V8 motor. On Wednesday of last week I was driving home and my check engine light came on. I stopped by Autozone to have them poll the computer for the error code. Misfire on cylinder 8. I immediately come here and start searching for common causes. Most results point to bad coil packs. I order 2 of the Accel ones immediately and a new set of plug wires. Friday afternoon I have my parts and install them. I let it idle in the driveway and a few quick blips of the throttle. Check engine light does not come on but the motor still runs rough. My parents came to visit this weekend so I didn't do anything further.

Monday morning (today), I fire it up and nothing has changed. No check engine light but still running rough. I drive it to work and when I'm about 5 miles away from work and the idle speed is getting really low, around 400RPM. The next stop light and it dies. I put it in neutral and fire it up shift to drive and go. Then at each stop from there to work it dies. Frustrating to say the least.

I had a problem 2 or 3 years ago with the idle air controller (IAC) which wouldn't allow me to even start the truck unless I had my foot on the accelerator. Even then as soon as I took my foot off of the gas it would die. I replaced that so I'm thinking that's not the issue here plus it's still running really rough. At this point I'm leaning toward the MAF. I was really close to pulling it here at work and dousing it with windex but decided not to as that doesn't seem to be the prefered cleaner. However I'm getting desperate so I may do that at the end of the day as I feel I've got nothing to lose.

 

Test the MAF by simply unplugging it (engine off). Then drive. If there is no change, the MAF isn't the problem. If it was bad, the computer will substitute basic parameters in place of the missing sensor and the engine will run much better.

FWIW, this doesn't sound like a MAF. Those usually idle okay, but stall or stumble when you hit the gas.

You did change the plugs while you were in there, didn't you?

Steve

 

I'll try unplugging the MAF. I didn't change the plugs this time, I did that around 30,000 miles ago, I'd assume those are still OK.

Thanks for the suggestion.

 

Sounds like your iac went bad again. You may also have a bad vacuum hose in the rear of the intake causing the rough running. If you unplug the iac when it is running, it should die or idle way down if it is working.

 

The fact that it's running really rough leads me to believe it's more than just the IAC. When that went bad it was still smooth it just wouldn't maintain an idle. I'll check the hoses for a vacuum leak. Thanks a lot for the suggestions.

 

This sounds very suspicious to me. Almost like you are having the infamous leaking head gasket failure. Usually the #4 cylinder will misfire, not the #8 though. I would immediately check your coolant level, pull the #8 cylinder plug and look for coolant in the cylinder.

You do NOT want to get the HYDRO LOCK sooo many '97s experience. Run a search for HYDRO LOCK and there will be TONS of information. Blue Oval probably will chime in, as he knows about this problem first, second, third hand, etc., etc.

Also, it takes some driving cycles to throw a misfire code so if you cleared the original code, it may take some miles, start/turn off the truck cycles for it to reappear even though the cylinder is still bunk. I cannot stress enough to check those gaskets!!!!!!!!

 

Beowulf, my coolant level is normal. I did a system flush last fall before the cold weather hit.

I made a mistake in my original post. I did not replace the plug wires until this afternoon. I went straight for #8 and replaced him first to see if it would help and it did. The motor runs as smooth as it did prior to all of these issues. A small victory I think.

Now I still have the failure to idle. Perhaps I can elaborate on that a little bit more. I crank the motor in my driveway while the truck is in park. If I do not touch anything it will sit there happily idling away. If I tap the accelerator ever so slightly, say I bump the motor up to 1200 RPM, and let go the revolutions fall through the floor and the motor nearly stalls. I think it only stalled once out of several attempts. After that short throttle blip and RPM nose dive, if I just let it sit for a few seconds it will resume a proper idle speed.

I think I'm still looking at a vacuum leak. Thoughts? I couldn't trace it down to any hose in particular. Will AutoZone carry all of these hoses or will I have to talk to the Ford dealer?

 

Now I think it might be related to the EGR system. If I unplug the vacuum from the EGR valve the motor will idle just fine. I took off the EGR valve and took it to AutoZone. There the parts guy put a vacuum pump on it, pulled vaccum, and it held. He suggested it might be the DPFE or the vacuum regulator that feeds vaccum to the EGR valve. The strange thing is that I'm not getting a Check Engine light for any of this. That makes me think it's something mechanical.

I'm going to clean the EGR and put it back on, then I'll check the DPFE and regulator. Other than that I'm running out of ideas and losing faith in my truck.

 

Update: Plugging the vacuum line didn't do anything today. I cleaned and reinstalled the EGR valve, reconnected everything, started the motor and let it come up to it's regular operating temperature. Just sitting there it still idles just fine, tapping the gas causes the revs to go up and then fall through the floor and stall the motor. It does not make any difference if the vacuum line to the EGR valve is open or plugged. VERY frustrating.

 

Another update: I ran some seafoam through it and cleaned up the throttle body. It seems to be OK now. :o