Join Date: Sep 1999
Location: San Diego, CA
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Re: MPI Kit - Pinging & Timing
It sounds like you have run into a fairly common problem with the kit. If you're pinging too much, it could toast your engine pretty easily. There are some things you can do, but each has a cost/benefit.
More Fuel: This is probably the easiest and most effective thing to try and correct. The computer is set up to feed a 4.0 engine and you have a 4.2. You can increase fuel by 1) putting in higher flowing injectors, 2) getting higher fuel pressure to the fuel rail via a adjustable fuel pressure regulator or modifying your existing regulator for more fuel, or 3) make or buy an adjustable MAP sensor. The computer shouldn't allow the engine to run too rich unless the amount of fuel is cranked up so much that the computer can't restrict it any more.
Adjust Timing: This will definitely help prevent pinging, but it takes away from performance proportionately. You can't adjust the timing by turning the dist. The computer controls when the spark is sent. You just need to make sure the dist is lined up to deliver the spark when the computer sends it. To make sure you are in the range, while the Jeep is running turn the dist until it starts missing a little (note the position), then turn it the other way until the same thing happens (note the position). This is the range. You should have it basically in the middle of that range so when the computer decides to send the spark, it is lined up. There is also an indexing tab in the dist that can be cut off to true up the phasing, but that really isn't where you need to go at this point. The best way to really adjust the timing is through the computer itself. You need to be hooked up to a DRBII type scanner to adjust it and I believe it can be retarded 4 or 6 degrees BTDC. Another way to adjust the timing, but not ideal is to shim the crank position sensor mount a little bit in the clockwise position. This tricks the computer as to what TDC is and gives the affect of retarding timing. This works OK within a few degrees, but done too much it gets out of line with the cam position sensor (which takes us back to the tab in the dist).
Run a 4.0 head: The head design is better and less prone to pinging. You want to make sure the head you use hasn't been shaved too much. This head is a much better head and will have a double or triple benefit to you.
Decrease Compression: This isn't the easiest, but it may be the best if the increased fuel doesn't do the trick and if you are already running a 4.0 head. Your head may have been shaved too many times or too much and has resulted in more compression. You can start with another unshaved head, you can polish the combust. chamber to increase cc's, you can make sure you aren't using too thin of a head gasket, etc.
Use a slightly longer duration cam: Even though Hesco will tell you to use a stock cam, you might benefit from a cam with slightly more duration to reduce the effective compression. The longer duration will increase the overlap and allow some pressure to escape. I would only do this if you are running a 4.0 head as well.