'94 B-4000, 113,000 miles I bought two months ago. I've posted before about the terrible spark knock. Yesterday I decarbon the engine with 3 cans of PM-3 (per the tech service bulletin here in Ford-Trucks). After I'm finished I need to go to Raleigh. The truck is running great. On the highway, doing about 70 mph, about 10 miles from home, all of a sudden the engine starts spark knocking like crazy and is losing power. After about 10-15 seconds of wondering if its going to blow, like a switch being turned off (or on) the problem stops and the truck takes off like a missile. Then 10-15 seconds later it does it again. The problem persists (off and on) the entire trip. When I get to my destination, the truck sits for about 5 hours. On the way home and this morning, no problem (still spark knocking like before). Today I'm cleaning the MAF and throttle body intake. This was too obvious a problem to just blow it off and hope it doesn't happen again. Suggestions? Thanks.

 

Pull the codes. Here's how it's done without a scanner on an EEC-IV system like yours:

http://www.shophoenixproject.com/eec/eec.htm

Good idea to clean the MAF.

One thing to consider also is a fuel delivery issue. You might start with changing out the fuel filter and try and do a fuel pressure check if possible.

 

How are plugs and wires?
Does sound like a fuel issue.

 

Plugs and wires are brand new (before the MP-3). I plan to change the plugs and oil/filter after all this internal cleaning. Where is the fuel filter. Is there any tricks to changing it?

 

Ingesting large amounts of PM3 to decarbon or spraying lots of cleaner into the intake results in that crud landing on the spark plugs and fouling them. It causes a coating of conductive material to build up which weakens spark. You probably need to replace the plugs (again).

The fuel filter is on the frame rail, left side, and approx under your left foot when you are in the drivers seat. There is a c-clamp holding it to a easily removed braket.

 

Yeah. I believe the procedure on FTE said that. Buy cheapo plugs or do this around the time you replace them. PM-3 does/will foul your plugs when this is done. Ruin the cheapy ones, then put new ones in there that are your real choice. Problem (should be) solved.

 

I like your idea to clean the MAF & throtle body.
I'd suggest you remove it to do so & be sure to use a non residual spray cleaner, like throttle body spray ceaner, for the MAF.

I also like Rockledge's ideas to check the computer for codes, check the fuel pressure & change the fuel filter.

If the filter hasn't been done, it's WAY over due at 113K & weak fuel delivery as Ranger Pilot suggested, can cause a lean mixture & ping!!!!

You may need a special tool to release the fuel filter pressure fittings from the fuel lines, so check your autoparts store when you get the filter.

Also check the "Technical" thread atop this forums main page, on how to SAFELY change the fuel filter & clean the MAF & throttle body.

Billy is right, the PM-3 may have fouled the new plugs with all the stuff it broke loose, so check them out.

Changing the oil & filter at this point is also a good idea, as the PM-3 likely put plenty of stuff in the oil & filter, so I wouldn't delay in changing it, as Ford also reccomends doing so, after the PM-3 treatment!!!!

I think your doing some good things, to bring the truck up to date, on scheduled maintenance, although I'd have done them in a little different order, like pull the computer codes first, to get some clues of where best to begin the spark knock fix, then maybe clean the MAF & throttle body, then do the fuel filter, then the PM-3 treatment, then change the plugs, wires, oil & filter last, as the throttle body cleaning & PM-3 treatment can dirty up the plugs, oil & filter.

I too have a 4.0L & it's even persnickety about the brand of gas I use, so be mindfull of that & try different brands of gas where you live & see if that helps some.

Anyway keep at it & let us know how it goes.

 

My 4.0L is picky about it's gas as well. It's a demanding engine lol.

 

also when you change the plugs i would stay away from the bosch plats. i have had good luck with them on past BIIs with 2.9s.but that was several years ago. i now have a 86 ranger with a 2.9 and used them on it and have had nothing but problems. also some of my friends that used on other ford engines have not had the best results.

 

Yeah I heard Bosch's weren't real good with our engines. I run Motorcrafts as I hear lots of good things about the plugs and wires, but expect to pay for the brand name. Also, I want to say that it changed from double plats to some other type of reccommended plug. It's been a while, sorry...

 

If all the fuel rail/spark plugs don't work, check your coil/electronic spark? module. I had a similar problem and it turned out to be the electronic ignition/coil

 

For the life of me I can't find the correct gap for the new plugs (Motorcraft Platinums). I've searched here and other sites and found everything from .035 to .052-.056. The manual says nothing about the gap. I trust ya'll most. Any ideas?

 

Correct range. Set the gap at the lower end.

 

Thank you Cowboy. You've been a big help with this truck.

 

It should be in your manual under spark plugs.