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Old 10-01-2019, 03:15 PM   #1 (permalink)
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87 258 obd1 mpfi spark knock HELP

I have an 87 yj with a 4.2 in it. I'm running MPFI and head off of a 94 yj that had and auto tranny. My problem is it's detonating/ spark knocking at part throttle under load(such as large hills). Since its a later model 4.2 it has 9.2:1 compression vs the 4.0 with 8.8:1, so I thought I'd just have to run 93 octane. I filled it up with 93 the next day and it helped a little but its still there! Any ideas on what I can do to retard the timing or if I did something wrong?? It's my daily and I really don't want to pop a hole in a piston AGAIN....
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Old 10-01-2019, 04:04 PM   #2 (permalink)
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A friend of mine had a huge problem with that with his Mopar MPFI swap. His problems went away when he swapped to a 4 L head which you already have. Have you put an adjustable fuel pressure regulator on it? His did lean out early on.

At the same time of the head swap he did put a header on it and free-flowing exhaust.
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Old 10-01-2019, 04:21 PM   #3 (permalink)
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A friend of mine had a huge problem with that with his Mopar MPFI swap. His problems went away when he swapped to a 4 L head which you already have. Have you put an adjustable fuel pressure regulator on it? His did lean out early on.

At the same time of the head swap he did put a header on it and free-flowing exhaust.
I have all of those. My pump does 70 gph and the adjustable regulator I have does 43 psi no vacuum and 33 psi at idle. Magnaflow muffler, stainless header...
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Old 10-01-2019, 04:38 PM   #4 (permalink)
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I donít know if it matters but at some point he did swap in the hesco cam. I donít know which one it was but it was California legal with mpfi.
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Old 10-01-2019, 04:49 PM   #5 (permalink)
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I don’t know if it matters but at some point he did swap in the hesco cam. I don’t know which one it was but it was California legal with mpfi.
...it can. The more valve overlap the less compression the engine is able to create...I reallllyyy don’t want to do that but it’ll have to be on my list considering I’m running a stock 4.2 cam. It can mess around with valve timing in comparison to injector/ spark timing.
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Old 10-01-2019, 04:52 PM   #6 (permalink)
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Iím just trying to figure out whatís different about your set up from his. His knocked and pinged and rattled like crazy for years. Now it drives beautifully.
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Old 10-01-2019, 05:20 PM   #7 (permalink)
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Iím just trying to figure out whatís different about your set up from his. His knocked and pinged and rattled like crazy for years. Now it drives beautifully.
Well I have the stock cam, 7120 head, all brand new sensors and distributor, rc9yc plugs, new plug wires, header, early model 4.0 intake, adjustable fuel regulator, external fuel pump that does 70 gph, and stock fuel filter. The regulator is set at 40 psi, and i removed the vac line running to it so its ALWAYS at 40psi.
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Old 10-02-2019, 12:56 PM   #8 (permalink)
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is Hesco's website still up ? used to have a pretty good MPFI board...
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Old 10-02-2019, 02:53 PM   #9 (permalink)
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is Hesco's website still up ? used to have a pretty good MPFI board...
Their forum? I tried to join so I can read the posts.... still waiting. Lots of parts and forums that I’ve read are from 2005 and lead to dead ends....
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Old 10-03-2019, 05:59 AM   #10 (permalink)
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I had that problem with the MPFI kit on my 1986 CJ7. I ended up spacing the pickup by the harmonic balancer with some washers. Hesco will adamantly tell you not to do this, but it worked on mine and a lot of other Jeep's with their kit.
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Old 10-03-2019, 09:30 AM   #11 (permalink)
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I had that problem with the MPFI kit on my 1986 CJ7. I ended up spacing the pickup by the harmonic balancer with some washers. Hesco will adamantly tell you not to do this, but it worked on mine and a lot of other Jeep's with their kit.
I beat you to it! I’m running an ax15 with 4.0 flywheel so I have a factory located crank sensor. I slotted it out to retard the timing. It originally sat at 12* at idle. Now it sits at 8*. Running 87 octane with no noticeable ping! I have a slight exhaust leak so I can’t 100% confirm it’s safe on 87 octane due to exhaust sounds coming from the mid section of the Jeep (I’m fixing it in 2 days). I can definitely say that it is practically gone.
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Old 10-03-2019, 09:35 AM   #12 (permalink)
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Other things I have done.
1. Modify the oil baffle on the vacuum side of the ccv.(cutting and bending the baffle down and away from the orifice)
2. moving my danky cone air filter away from the driver side of the engine bay to the far right passenger side (almost a 30*F cooler filter temp I got with a laser thermometer)
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Old 10-03-2019, 02:56 PM   #13 (permalink)
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how fresh is your motor? get any oil in the TB and it will ping...had 30,000 on mine when i put the mopar kit on it...did the 7120 head and jba header about 38,000...run like a top after that
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Old 10-04-2019, 12:28 PM   #14 (permalink)
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A Chrysler DRBII or DRBIII factory scan tool allows up to 6 degrees of retard to the ignition timing, in 2 degree increments. Not sure if a Snap On Modus scanner would do that too.
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