1998 F150 4.2l V6

Got this truck in a trade a while back and have been doing some work on it (complete front end replacement, shocks, coil, wires, plugs, fuel filter, etc.)

It has never run "great" and was hard starting, but took a turn for the worse and would randomly not start at all. Found there was sometimes no fuel pressure, and checked the fuses and relay, and finally replaced the fuel pump and hangar assembly. Also found a burned vac line at the back of the manifold and replaced that. That fixed the no starting issue, but it's far from fixed.

After replacing the fuel pump, it ran fairly well for about 15 miles, then started missing and stuttering again, and got progressively worse for the next 40 miles while getting it back home. I found the intake snorkel was loose (ok... off...DOH! brainfart after spraying some carb cleaner while trying to get it to start before changing the fuel pump)... put that back on and tightened it up and figured that was my problem...but it actually ran worse afterwards. Using a cheap phone app, I saw the fuel trims maxing out again. Assuming another major vac leak, I smoke tested it today after work, and see a small bit of smoke coming from somewhere under the upper IM, but nothing major. I just reset the ECU and let it idle a bit, and now I'm seeing very high to maxed out long term FT again on bank 2, (but now bank one isn't horrible):

(idle, just before shutting it off)
Bank 1 STFT = 10.2%, LTFT = 6%
Bank 2 STFT = 10.2%, LTFT = 19.5%

I've pulled the intake and inspected the MAF, and it looks great...no build up and both wires look clean. The only DTC I'm getting while idling it now is bank 2 too lean. (It used to occasionally throw both at once, at any load...if anything) It acts like a massive air leak somewhere, but using a smoke machine I don't see strong evidence of that. It's also weird that both banks were running very lean and pretty much even, and now bank 2 seems to be the main culprit. The only other thing I can think of is fubared injectors or an electrical gremlin (grrrr).

Any ideas?

FWIW - I'm pretty familiar with tuning and such (wrote some software for tuning DSMs)... but I'm a Ford F150 newbie and clueless on these engines/ECU's and all their idiosyncrasies. And unfortunately I'm really limited on what I can see going on.

 

97 and earlier 98 4.2's had intake and front cover gasket issues. I'd pull the intake, plenum, and front cover off, clean them up, and replace them with the newest gaskets. You will need to have the engine build date, 1/15/98 things changed. It's on a sticker on the driver's side valve cover. If it has a IMRC, that needs to be checked too.

 

No VC sticker...just some residue where one once was.

Date on door jam sticker is 03/98

 

That could go either way. I don't know how to tell whether it's an early or late engine by looking at it.

Does it have an exhaust leak before the upstream O2? That will cause lean codes.

 

I don't hear anything, but haven't checked closely.