1993 F140 4.9L - Bad Miss When Warmed Up

I recently was given a 1993 F150 4X4 with the 4.9L I6 and a MT. It has 172k miles on it but did not run when I received it (has been sitting for 2+ years).

Diagnosis indicated a no spark condition which was remedied by repairing the wire terminal on the negative side of the primary coil connection. Combine this with a set of spark plugs and it started and ran (albeit poorly).

Further diagnosis indicated that the ECT was in need of replacement. The ECT was replaced and the truck runs fine until it warms up (closed loop) upon which it starts to miss badly and smells rich. It also pops through the exhaust and has already ballooned the muffler somewhat.

I have checked/adjusted the timing to 10deg BTDC with the spout jumper removed and have gone over all of the wiring connections and grounds with the exception of the injectors and 02 sensor.

After running the engine until it reached operating temperature, I performed a KOEO test and found that I only have 2 codes: 522 (I did not have clutch depressed) and 111. The 111 code was in CM.

I have verified that I do not have gas in the vacuum line of the fuel pressure regulator so I do not think that that is the issue. In addition, I checked all of the vacuum lines and intake area for a vacuum leak (with brake clean) and did not find anything out of the ordinary.

Please note that I have not driven the truck yet as I have to replace a few rusted brake lines after I get it running correctly.

Any ideas?

With the engine running overly rich as a result of operating w/ a bad ECT sensor, there's a good chance a clogged Cat could be inhibiting proper valve closure. That would explain all symptoms mentioned. Anything over 3psi would indicate a back pressure condition. A muffler shop will usually check it for free!

It may sound odd but i've had a 5 liter f150 have the same symptom with a bad IAC. Normally a back pressure issue is seen as a loss of power. Just curious, Have you managed to check/change the fuel filter and/or gasoline? I noticed you said the truck had sat for 2 year. Just a thought, I have seen crazier things.

I forgot to mention that the previous owner had the catalytic converter removed. As such the smog pump is still intact but the hose that led to the cat has been crimped off and welded shut. Also, the fuel is still the old fuel.

I will check out the IAC as I received the original with the truck. I am also going to chase all of the vacuum lines tomorrow and may try a can of Seafoam for good measure as well.

If vacuum lines check out, I will fire it up and start pulling plug wires to see if I have a bad wire or a stuck injector (note that I did install a new set of Motorcraft Platinum plugs).

Thanks for the suggestions, I am looking forward to more.

There's all kinds of things it could be. Almost allof them are not to serious really. The thing about a truck that you don't know the history of, is you really don't know what you have. There is a tendency to start hit & miss tinkering w/ it. $25 here $15 there another $50 for something else. A dry & wet compression test will tell you right from the start if you might be beating a lame horse and how much time & money you should or could invest (or may be throwing away) The tests will tell you the basic health of the engine, weather the compression is balanced throughout the cylinders, conditions of the rings & valves. Alot of the problems after those are ruled out are just makeing up for previous poor maintanence. You'll be able to decide to do the minimum to get a few more miles out of it or this, which greatly improve performance & MPG at the same time (about $140): FullSizeBronco.com - 78 - 96 Ford Bronco offroad club, forums, tech, installs
(http://fullsizebronco.com/forum/showthread.php?t=63592&highlight=timing+bump)

With your added info, my next guesses % wise would be a dirty IAC or a hanging injector. Or dirty injectors in general causing poor fuel atomization. Poor fuel quality as mentioned, sure wouldn't work in your favor. Two of my favorite products are Lucas Injection cleaner & Seafoam. There is always a noticeable improvement & the Seafoam is fun to use (you'll see what I mean!):How to Seafoam an Eighth Gen F150 (1987-1991) - Ford F150 Forum - Community of Ford Truck Fans (http://www.f150forum.com/showthread.php?t=31505)

Just wanted to update everyone. I finally had a chance to verify the integrity of all of the vacuum lines using a vacuum pump - everything checked out fine.

I removed the IAC valve and cleaned it without any improvement. I then replaced it with the original one that I received with the truck without any improvement - so I re-installed the one that I cleaned.

The next thing that I did was verify proper operation of the EGR. The engine died at idle when I applied vacuum.

I also swapped EVP sensors without any improvement.

I put 1/2 can of Seafoam in the tank (currently at 1/4 full). I ran the engine for about an hour this evening. I have noticed that the truck seems to backfire through the exhaust at idle, when you hold the throttle open (around 2k rpms), and when it returns to idle after revving it up. With that said, after you hold it at around 2k rpm for a bit, it seems to "clean" itself up for a brief period of time.


In addition, I pulled individual spark plug wires and each time the motor stumbled as expected.

After scratching my head for a while, I decided to pull the vacuum from the MAP sensor. Low and behold, the backfiring completely went away although the engine now idled at around 1400 rpm and ran on the rich side based on the exhaust fumes. With the MAP vacuum disconnected the engine idled smooth and I could rev it up without having it backfire through the exhaust when returning to an idle.

Based on these observations, I am suspecting that I have a bad MAP sensor. Is there a way to check these devices? Do you guys think that I am on the right track?

Your just changing the Vac system modulation by removal of the Vac hose. Disconnecting the wires to the map would eliminate the MAP input. I would introduce Seafoam thru the air intake manifold How to Seafoam an Eighth Gen F150 (1987-1991) - Ford F150 Forum - Community of Ford Truck Fans (http://www.f150forum.com/showthread.php?t=31505)

First things first, get that old ass gas out of there. That stuff can cause all kinds of crazy problems. It probably isn't burning properly which the O2 sensor is seeing as a rich condition because it is going into the exhaust as raw fuel, old raw fuel lol. Second, it is probably pre-igniting causing some detonation which the engine retards the timing for, which also causes it to run rich and pop through the exhaust. Retarded timing also means low vacuum especially if the engine is popping through the exhaust due to the timing. Low vacuum calls for more fuel. Disconnecting the MAP sensor kind of helps because the engine controller defaults when it sees atmospheric pressure. Last but not least, seafoam won't fix bad gas. Bad gas can also burn valves, popping through the exhaust is a sure sign of that happening.