Back in November I bought a 1990 F150 XLT Lariat with a 4.9 EFI, a five speed, and 148K on the clock. Had really good power...pulled hard...but seemed to have a slight miss/rough idle. There was an occasional "PFF...PFF" sound from the exhaust and the motor shook slightly. PO said he got the truck from his dad back in '98 and it had been doing that as long as he had it. It didn't affect the driveability of the truck, so I didn't think much of it.

Here's what I did:
-Removed upper intake manifold and replaced gasket, replaced valve cover gasket, cleaned throttle body and installed new gasket, thoroughly cleaned IAC/ EGR and replaced both of those gaskets.
TPS seems to be in good shape. No gunk, no play, good connections.
-Replaced pushrod cover gasket, then installed new plugs, wires, dist. cap, and rotor. I did not disturb the distributor position.
-Cleaned all of the plugs and terminals with terminal cleaner. Torqued everything that needed to be torqued...went back and double checked that, and everything was within spec.
-Changed oil and filter. PO said he had always run 20W-50, so that's what it got...along with a Motorcraft filter.

Now when I crank it up it starts immediately, goes into high idle for only about 5 seconds, and then drops into low idle. At random, there is still a "PFF...PFF" sound from the exhaust and the motor shakes slightly. It is not consistent, as in a bad cylinder or a misfiring plug. At times, the idle seems to be wandering...RPMs vary by about 150.

When I slowly depress the accelerator, the engine will hesitate ever so slightly for a second, then smooth out. If I punch the accelerator, there is no hesitation...it revs right up, then returns to low idle...runs smooth for about 20 seconds, then eventually the "PFF...PFF" returns to the exhaust and the motor shakes slightly.

It's worse when it is cold than it is once it's warmed up completely. Oh, and I have no check engine light (yes, it works). I have purchased the Equus OBD-1 code reader for Fords and will begin pulling codes as soon as I get the new heater core in (old one started leaking BAD last week). But if there was something really wrong with it, wouldn't I get a CEL? I have a 2000 Nissan with 115K that will seemingly throw a code and a CEL if I look at it the wrong way...

Does anything stand out to you guys...as in something I did wrong?

I have come a long way on this truck, but still have a long way to go. Wheel cylinder blew out on Friday, so I now have two new ones sitting in a box ready to go on along with new brake shoes. Clutch slave cylinder is leaking, and I am hoping to make it till summer before I replace that. I have already replaced the leaking rubber plugs on the trans top cover with steel expansion plugs, rebuilt the shifter, cleaned out the case, and filled it up with new Mercon V. I'd like to actually drive it for awhile before I drop the tranny out.

 

Cracked exhaust manifold?
Bad or burnt exhaust valve?
Have you checked compression in all cylinders to rule out cylinder compression loss?
If plug wires aren't new, have you checked each to rule out possible shorting out?

Usually cold start-up will have the high idle, my 4.9 does and I have heard of a lot of people with 4.9's that have the similar issue. I simply goose the accel to trick the PCM to dropping it.

As far as oil weights go, I think that 20w-50 is a bit heavy for the engine, especially for cold weather performance. Just my 2 cents, so don't criticize me to much, some may agree or disagree. I don't see where it could cause any problems, except for it's abilty to get to the top. 4.9's have a harder getting oil to the top than other motors do. I have read somewhere about this, unless I have misunderstood some of the the post from years past. My 4.9 always ticked like crazy when I 1st started it and I have tried every oil to remedy it. No avail. I just recently installed a pre-oiliing system with a pump to prime the filters and the block and it never has done it again.

If after you did all this work, you had previously disconnected the battery, then reconnected it. When you 1st started the truck and verified nothing had gone terribly wrong, you drove it long enough to let the PCM re-learn it's driveability factors?

Maybe someone else can chime in with some other ideas.

 

id check everything he ^ said. my 4.9 always has a high idle whenever its a cold start too. im thinking the 20w-50 is a little much. but ive never ran it. ive always used 10w-30 in mine.

 

mine seems to do exactly what yours does. my emissions air manifold leaks where the log splits into the small runners that screw into the head. take a good look at that piece. it runs next to the fuel rail, and seems to be a bad place for an exhaust leak. I'll be changing mine out in the spring. it looks like a repair that I should wait for some nice weather for...
I get a cel for mine, but I'm obd-II

 

Bad cables/spark plugs will cause the same condition if they aren't quite gone yet.

 

While you're there, check the ignition timing, the 4.9 loves to be about 14* BTDC

 

What cylinder is giving you the problem?

Dave