i have a bad preignition problem with my '93 4.0 v6. I have cleaned the Air Temperature Sensor, and tuned it up. I also tightened the lower intake bolts and two were pretty loose. it still pings a lot. it is "undriveable" on regular 87 octane and pings on premium. I checked the codes and it is normal, no problems showed up. i know about the decarbon process and i may have to try it but i suspect a vacuum leak. the engine surges at idle. not much but a tiny bit. most people wouldn't even notice but I do. with the vacuum gauge hooked it shows the surge also. it is very slight but present.
i was told to try a propane bottle with a hose and direct that around in the engine compartment to find a vacuum leak. it does not work. i finally pointed the propane hose right into the air intake and then the engine idle did raise but it never made a difference when i directed it around the intake manifold. does anyone have any suggestions as to where the vacuum leak might be or what other problems could be causing this?? i do suspect the intake but without proving there is a leak, i hate to go through the pain of replacing it.
i would appreciate any suggestions or comments anyone has. thanks very much! Kent

 

If you just can't find a leak there may not be one. It could be other things causing the pinging, check some threads. Also If you are sure its a leak the dealer has a smoke testing unit the can find just about any leak. It may be time for a trip to them.
Dave

 

Most common reasons for ping/knock on the 4.0L (in no particular order):

1) Engine needs to be decarboned
2) Dirty MAF sensor
3) Loose lower intake bolts.

You can check for a vaccum leak by first looking for loose/damaged hoses. Next, spray carb cleaner on mating surfaces of the intake, etc. Remember to check the seal on the gas cap for damage and be sure the cap is on tight.

It would also be good to pull codes. Since the timing is factory set and is managed by the computer, I would not go that direction.

 

As suggested, first thing is to pull the codes. Unlike the newer OBD-II systems ('95+), your '93 EEC-IV system will store trouble codes without necessarily setting off the CEL. Address any codes that may be present. If your hunch is correct, you should get a code or two indicating a lean condition. That would indicate an air leak, or maybe a fuel delivery issue.

Clean the MAF sensor and decarbon the engine, too. When's the last time the engine had a tune up?

 

i pulled the codes and it only had a code "1 1 1 " which means systems pass. I just gave it a good tune up about 2000 miles ago and that helped a tiny bit but only temporarily it seemed. I still think it must have a vacuum leak because of the way it surges. guess i'll try carb cleaner as was suggested but it's so darn hard to even see the intake let alone spray anything on it. I did clean the maf sensor if that's the one that is in the air cleaner. i'll keep looking for a vacuum leak and if i can't find one, i'll try the engine decarb. thanks, Kent

 

Might be time to check the fuel presssure. Lack of fuel can cause the same lean condition as too much air.

 

To all the previous good suggestions I'd add, check the EGR & it's control system.

If it's not controlled poperly or is clogged up & misbehaving, it shure can make one knock like crazy.

 

Does it have an automatic tranny? Vacuum leak could be way down there too. That happened to my ranger, tried like heck to find the leak with starting fluid, no luck...It was under the truck...the metal part of the vacuum line had a hole in it.