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?

 

check your throttle body gasket. Mine was partially "sucked" in and leaking air. It caused a stumble/miss feeling under light acceleration.

 

<table id="post6071450" class="tborder" width="100%" align="center" border="0" cellpadding="6" cellspacing="0"><tbody><tr valign="top"><td class="alt1" id="td_post_6071450" style="border-right: 1px solid rgb(209, 209, 225);"> 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?