Hello,

Looking for some help on an idle and running issue.
I have an '88 f150 300 straight 6, 4wd with the 4 speed Berg Warner tranny.

When I start the truck it goes through about a 10 minute cycle of idling rough. It will start up, hunting for idle for a couple revs. Then it will idle at 1300 rpms for about 30 seconds before climbing to 2000 rpms. Once it reaching 2000 it will hold for about 15 seconds and then drop down to 1000 rpms. It will then backfire for about 10 minutes until it revs up as if it gets a jolt of power. It will then stop popping and idle fine. That's the idle issue. Also, I should probably mention that it runs like **** if I start driving it before it goes through this cycle. It feels like it might be missing if I do: low power and lots of backfiring. Even sometimes when I'm just driving at a constant speed.

From a stop, it will hesitate when I hit the gas. It doesn't do this all the time though. Only from and stop and only when I first hit the gas. No apparent pattern on when it does or does not hesitation, although it it is roughly 60% of the time. It will even alternate between hesitating and running smoothly within one drive. It will also backfire if I take my foot completely off the gas but keep it in gear. Take it out of gear, and it stops.

My oil pressure gauge does read low, but I was told that with the way the pressure switch works on these trucks the oil pressure is fine. I have yet to plug a gauge into the block though. It will fluctuate between "N" and close to the lower line.

My exhaust is broken off after the muffler and there is a crack on the weld into the muffler. I'm sure this isn't helping the backfiring, but could it really be accounting for this much backfiring? It also doesn't make sense to me that it stops backfiring when idling if I let it sit long enough.

I have replaced:
-Plugs
-Fuel Filter
-Tested TPS (Good)
-EGR Valve
-Cleaned and tested IAC (Good)

Sorry for such a long explanation. I figured the more you guys know the more you can help. Any ideas on what could be causing this would be much appreciated. I've been trying to work this out for several weeks now. Thanks in advance

 

Do a compression check. I think youve got bad rings, or a hole in a piston. this is how my 1993 5.0 was acting, and we found one of the pistons had a hole in the top of it.

 

I usually relate backfiring to a timing issue (Timing Chain, Dizzy), or leaky valves.

What's the condition of your Dizzy Cap and Rotor? I would replace. Keep your plug wires separated from each other.

Put a timing light on it. Check your base timing. Then watch your timing to see how far it is advancing when backfiring.

 

Okay, I will work on getting a compression gauge to test and I will inspect my dis cap and rotor more. They look okay at a glance, but I will do some real testing.

 

Ended up taking it to a shop. The oil pressure was good, but my sending unit was bad and leaking. They are going to replace it--hopefully this helps. Should I still have them run a compression test or would this be redundant knowing that my oil pressure is good? Don't know how much overlap there is between these two tests.

 

No overlap on the tests.

Could the problem be with the IAC, Idle Air Control valve? This sounds to me like an idle control / computer issue and not an ignition, or internal issue. Maybe the sensors in the intake tract need a good cleaning and adjustment so they can give the fuel system better info?

 

I already cleaned and tested the IAC and it checked out. I also tested the TPS and it was also good.

The shop said they found a bad seal on my distributor cap and they suspect moisture was entering and causing a miss. I am having them replace the distributor cap and a few other ignition components. I'm hoping this does the trick. If not, I will get a compression test done.

 

You might consider changing your fuel injector seals as well.

 

Got the truck back a couple days ago and I'm still having the same issues. I do think a lot of the backfiring has to do with the exhaust, but I am worried by the surging and hesitation from a stop. This seems like fuel delivery to me. My injectors aren't leaking from what I can tell. Do you think this could all be related to the injector seals?

 

This type of issue is tough to diagnose but it could be a pickup coil going bad. It's at the bottom of the dizzy if I remember correctly. Had a similar issue with a 93 with the 4.9 and that was the solution. It was after swapping injectors that I tried this... good thing for parts trucks.

 

I relate the 2000 rpm to Injector Seals, or some other vacuum leak.

Did you do the compression test ??

 

Put a timing light on it. Check your base timing. Then watch your timing to see how far it is advancing when backfiring

 

Like he said, cleaning the ACT in the intake is always a good idea.

 

Is the ACT the same as the IAC? If so, I already cleaned it. I just bought a compression tester today, but I won't be able to get to testing for a couple days. I went ahead and bought the injector seals as well, so if compression is good I will probably replace those. I don't have much experience with checking or adjusting timing, so I will have to go buy a light and brush up on the procedure.

 

All of the distributor and rotor were replaced, so would the ignition coil have been replaced too?