The 4.0 v6 in my 1990 XLT E4WD just started to run rough, sounding like at least one cylinder is missing. The missing seems to be worst when trying to accelerate, and there is an associated loud pulse. There is also a slight roughness when idling. It happened rather suddenly, so I'm thinking something just failed.

I ran the self-tests, but it showed no real errors; brake and goose errors during KOER, as I didn't know when to apply those.

All but one spark plug show lean condition; cylinder 6 is the only one showing the normal light reddish brown color. The others are showing white residue over everything. So I'm thinking it's not getting enough fuel. I changed the fuel filter, which did not help. I can see the static fuel pressure comes up to 40 psi with the key-on, and at idle, but I can't see it under load.

Or possibly cylinder 6 is not firing, and sending raw fuel out the exhaust, causing the loud pulses. But I would expect the plug to be wet if it wasn't firing.

I changed the EDIS module, which also did not help. I'll try to find a replacement coil pack next, but if that was defective, I would expect 2 cylinders to fail. Though the running condition might indicate more than one bad cylinder, only cylinder 6 has the different plug condition.

Any ideas as to what to pursue next would be greatly appreciated.

thanks for reading.

 

i had an intake valve spring break on my 4.0.

 

Did that make a lot of noise? Like P to V contact?

I'm not hearing any valve train noise.

I replaced the coil pack, but that didn't change anything either.

 

did you replace the one spark plug with a new one? These 4.0 engines kill spark plugs. Try a new spark plug..

Also the 4.0 needs a revised Intake gasket.
There is or was a Service Bulletin about the intake gasket being swallowed so a revised gasket part number was issued. I had similar symptoms in my 4.0 1997. The revised intake gasket resolved the problem.

 

only popping noise in the exhaust and very low vaccum.

 

if it gasps for air when you step on the gas and coughs loosing power, it is definitely the intake gasket. been there done that.

not very difficult but not easy either. Lots of disassembly, you will need special inverted torx sockets to remove 4 studs retaining the intake manifold. I don't know what they are called. My nephew did the job, I just cleaned the gasket surfaces. Injectors must be removed, EGR tube is a total pain to remove.

 

Jose, I have replaced the intake manifold gaskets before. The original was a one-piece, and I replaced it with Fel-Pro two-piece, and had to fill in an eroded part of one of the water passages. I did not separate the fuel rail from the lower intake, so I did not have to use the E-socket to remove those studs.

Zenish, were they sharp pops, or uneven pulses? As the engine is idling worse, the vacuum is getting lower. I fear I may be having the same problems. I will have to open the valve covers today to check.

 

xlt4wd90 are we talking about the same gasket and engine?

without removing the intake manifold, (the plastic manifold in the center valley of the engine), how did you get a gasket or gaskets under it?

 

My 1990 model has an upper plenum that's bolted to the stud that stick out from the lower intake, but they go through the fuel rail, which has the injectors sandwiched between them. When I removed the lower intake, I left the fuel rail bolted to it, so took that out as a unit. After disconnecting some connectors, I left the wiring harness on the lower intake:

https://photos.google.com/share/AF1Q...Zycm5RNTZTeWZB

And now, after a week of messing with it, I finally found the problem. You get another cigar for your suggestion to replace that one different spark plug. I removed the plug again to check again, and noticed that it is now wet, probably due to the number of tests I tried to run since I started this. This was odd, since it was dry after the drive home when the problem first showed up. So I checked it against one of the other plugs, and noticed its gap is a lot wider than the other. Looking closer, I realized that, this being an Autolite APP105, a double platinum plug, it was supposed to have a platinum button on both the center AND ground electrodes. On this one, the button on the ground electrode had disappeared, so its effective gap was over .075, the limit of my gauge. That's probably why it wasn't firing, or not firing well. But the plug on its opposing cylinder that shares the same coil, #2, looked just fine, like the other cylinders. I re-adjusted the gap to about .045, re-installed it, and the engine started and ran fine.

So I need to get at least 1 new plug, or a whole set of 6. But I'm now a little apprehensive about getting Autolite again.

 

the 4.0 is a spark plug monster.

in my '97 one plug is always cracking the ceramic insulator and stops firing. This happens between 6 and 8 months of use.

I complained to Autolite. They asked me to send that plug with a form they provided.

Their response? a box full of spark plugs including Irridium and Kryptonite, you name it, 6 of each. I love Autolite now. They set me up for 3 years!

 

I have about 170000 miles on the van now, and this was the 3rd set of plugs. But I've never had a failure like this before.

Were your cracked plugs on cylinder 6?

 

it has happened on the front plug driver's side, and the rear plug passenger side.

I know the engine well enough now that i can anticipate when the plug's ceramic insulation starts to crack.

 

im going to borrow your photos if i ever get brave enough to try to fix my 4.0.

 

What do you need to fix? Leaking intake gaskets? If the pictures can help at all, great!

I was hesitant at first as well, but I managed to slowly get it done. You have to be patient and remove all the cables and hoses that are in the way before you can actually get to the manifold. Working in a cramped engine compartment like that also requires strong hands and skinny arms.

 

when my valve spring broke the push rod damaged the lifter.i need to remove the head so i can replace the lifter and your pictures show me where the bolts are on the intake.