1998 4.2L v6 RWD, 166K miles

Just got the truck back together after an intake gasket install, and now have a rough idle. Here's what's been done:

New coil, plugs, and wires
New fuel pump and fuel filter
New (refurbished) injectors
New fuel pressure regulator
New (updated) gaskets on IM and upper plenum, EGR, TB, ISC, etc.
New PCV valve
Replaced all vac lines
Cleaned IM, throttle body, and upper plenum, checked IMRC vanes and actuators (bench vac test), new bushings for IMRC rods
Cleaned fuel rail, checked for leaks before bolting up plenum, etc. Rail has pressure at key on (but not measured).
New t-stat, heater hose connectors at fire wall, water pump pipe
New EGR valve (pipe is still in decent condition)

This all started with hard starts (sometimes no start), maxed fuel trims, and rough running...so I decided to replace the fuel system and vac lines...which led to injectors (which were horribly clogged). Figured while I was in that far I might as well pull the IM and clean everything and replace the gaskets. After putting it back together, it's now idling rough and hesitates on throttle stab, but has no CEL or codes. (MAF is clean and pulling the connection makes no difference). I've gone over everything and can't find anything I missed. Haven't measured vac, but I can feel it at the EGR line (and brake booster has good vac when I pull the hose). Did a smoke test on it and see no leaks anywhere (other than fog leaking back into the intake tube pre-throttle body from the CC vent as expected).

I've built a few cars, but I wasn't a Ford guy before getting this truck in a trade... and this has me stumped.

Any ideas greatly appreciated.

EDIT:

ST fuel trim on bank #1 is hovering around -11 to -13, and ST fuel trim on bank #2 is hovering around +11 to +13. They are almost mirror opposites of each other.

WTH?

 

What kind of coil, plugs, and wires are you currently using?

 

Recheck the plug wire fire order.

 

 

By your fuel trims, I'd be looking for an exhaust leak like a bad gasket or a possible broken stud on the exhaust manifold. Assuming it ran right before the work started a plugged up converter would normally be out of the question but you also state the injectors were fouled horribly. A converter issue may not be out of the question. A heat sensing laser probe would tell you the story on that or an exhaust shop. Might check the wiring to the O2 sensors while you're looking.

 

Motorcraft wires, AC Delco coil I think. Can't remember on the plugs, but they were popular on RockAuto lol

 

have you checked the fuel pressure with a gauge to make sure it's ok?

 

Don't have a guage right now (mine bit the dust), but when pressing on the schrader valve I see pretty good pressure. I'll try to pick up a guage and measure to be sure though.

 

I have more information, but not sure what to make of it...

I checked plug wire order, and it's good. On a hunch, I hooked up an induction timing light and clamped it over each wire. They are all firing evenly and in sync with the RPM, except #4... I'm seeing much faster signals in it than the others. I pulled the new coil off and put the old factory one back on, and see the same thing.

Is that normal for these engines? Some kind of waste spark setup only on one cylinder or something?

BTW - I pulled wire #4 and it barely changed the idle at all...maybe a hair more rough, but not by much.

 

FTW, check the connect that goes to the coil pack. Make sure all pins are there and none are bent.

 

The 4.2 is waste spark on all cylinders. 3 & 4, 2 & 6, 1 & 5.

Aftermarket ignition is hit or miss on these engines, best to stick with Motorcraft wires, coils, and plugs.

 

Found the problem (after verifying all coils are firing and plugs are getting spark)

Injector #4 isn't firing. Pulling the coil wire for every cylinder drops RPM, except #4.Used a stethoscope and can hear all injectors working except #4. But, no codes are being thrown.

Injector coil reads 12.4 ohms, which sounds just about right. Are these injectors firing on 12v? If so, I should be able to hook up a tail lamp (DIY noid light) to the harness connector and see it pulse... right?

 

Yes, you need a noid light or something like it to check for a pulse. You can use a standard 12v test light. However, for this price, why not buy a set?


Swap injectors with another cylinder and see what happens.

I wouldn't be surprised if the injector itself is shot, and the others may need a rebuild or replacement.

 

I measured the voltage on a good injector connector, and I got around 13v on one pin (supply), and around .6v on the other..which would be about right I think for an averaged switching ground.

But on the connector for #4, I get the 13v supply, but 0v on the other pin. So it looks like that somewhere between the PCM and that connector, the switching ground is being dropped...which SUCKS no matter if it's in the harness or a PCM driver chip.

Are the injectors on their own harness that plugs in somewhere (please please please ), or are they all bundled with a main harness going to the PCM?

*****

I'm still curious as to why I would have an injector not firing, and the fast plug signal from the timing light on the same cylinder. Seems too weird to be a coincidence.

 

I've been looking around at them, but there is confusion on the injector connectors. Most of those sets have "Ford TBI", which my truck most certainly doesn't have. I believe it is a Bosch-2 style connector.

EDIT: I see that set does include the Bosch-2

That's a two hour job.

All the injectors were replaced with refurbished ones. Looks like no signal getting to #4. See above.