Engine Missing under load - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 07-26-2002, 09:21 PM
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Engine Missing under load

I just finished installing a remanufactured engine...long sad story. Before the demise of the other engine, all was running swell on my 50K miles 85 CJ7, 258, 2 barrel BBD.
After completing the switch over, I can't seem to get it back to normal. The timing "appears" right but the engine is missing under load. The idle is pretty good.
New equipment: long block, distributor cap, wires, rotor, plugs, radiator, alternator, thermostat.
I have triple checked all the connections and they seem fine.
The timing, with the advance plugged is set to 9 degrees. When I reconnect advance the mark goes off the scale but the engine still idles ok.
I'll stop babbling for now. I did my best to install the distributor where I think it is right, but the Chilton's manual instructs me to look for some non-existent "matchmarks".
Any ideas?
Oh, lastly, my instincts don't have me replacing coil, carb...because this stuff worked before the migration to the new block.
Thanks..
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post #2 of (permalink) Old 07-27-2002, 07:32 AM
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Re: Engine Missing under load

Does it seem like a regular miss, as though one cylinder isn't performing? At what engine speed does it happen? When you back off the throttle, does it immediately smooth out? How is your fuel economy?
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post #3 of (permalink) Old 07-27-2002, 08:11 AM
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Re: Engine Missing under load

Not a regular miss, and not at specific rpm. I can go for a ride and make it seem like everything is ok as long as I stay out of range where engine is under load at lower rpm. For example, shifting into 2nd or third gear before you get vehicle speed high enough will result in missing while the engine stuggles to continue accelerating.
I have pulled each plug wire and confirmed each cylinder is there. If put back on the old plug wires and regapped the new plugs but no success. Quick question...I have new Bosch Platinum plugs. I am not really familiar with this plug so I hope these plugs are not the cause. Already threw away the old standard plugs.
I will spend today diagnosing everything so should have more statistics later.
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post #4 of (permalink) Old 07-27-2002, 08:42 AM
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Re: Engine Missing under load

Does it go away at higher RPM, or can it not reach higher RPM?

Missing under load at low RPM is usually secondary ignition. Being erratic, it's probably before the cap/rotor/plug wires/plugs. Check the coil and coil wire. Less likely would be the module.

It could also be a plugged or restricted passage in the carb. Was the engine apart long enough to have caused the gas in the bowl to go gummy?

Low RPM generally rules out fuel pump and filter, and air intake restrictions.
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post #5 of (permalink) Old 07-27-2002, 08:43 AM
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Re: Engine Missing under load

Under heavy load at low RPM the plug's ignition demand is at it's greatest.

Since it doesn't feel like any one particular cylinder it's most likely a problem common to all cyls.
Check your coil wire - could be loose or may have broken. Try swapping in another one.

Platinum plugs also have a much greater demand than real spark plugs. It's real easy to tell an engine has platinums in it with a scope - they all look like regular plugs that are fouled! Unless the engine was originally designed to run platinums in the first place they usually don't function right as far as the engine's concerned.

But like most of the scam aftermarket items, split fires, fire rings, extra gappers, intensifiers, multi-electrode, yellow, blue, pink, or green they function like the plug maker wanted - they got your money!

Some folks will say they've had great results from platinums, it ran better with them. That's because when they put them in they replaced something or fixed something that was already defective, so they ended up "less worse."

Notice the number of part numbers for platinums aren't nearly as many as real plugs. They use the concept of "one size fits all", often the heat range is wrong. Ask your wife how well "one size fits all" works for her.

Best bet - the plugs the engine was designed for - heat range will be right, and indexing will be in the proper position IF you torque them to original specs.
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post #6 of (permalink) Old 07-27-2002, 09:49 AM
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Re: Engine Missing under load

So far I think I am hearing coil, plugs and ignition module. I'll start looking in this direction. So far this morning I have been looking into carburetion.
Findings thus far...I do NOT have a O2 sensor! Who took it? No joke...no sign of it in exhaust manifold. Next finding....If I am looking at the correct stepper motor metering pins (I only see one pin...the other must be hiding). No matter what I do, the pin does NOT move. I even unplugged the connector in back of stepper motor and everthing just keeps running normally!
I have two set screw in the front base of this BBD. They are adjustable and the left one adjust rich/lean (I think). The are both out about 3.5 turns. I usually have to turn the left one in to 0.5 to pass the emission. The jeep idles like crap, but I pass with flying colors.
I appear to have good suction on the ported vacuum. Pinched of the charcoal canister...no change in idle.
Disconnected the 10"/4" vacuum switch, no change, but I guess this only kicks in at higher vacuum than I have achieved driving around the block.
What else...my pulsed air system is gone. I knew about this one though...the tube to the catalytic converter was pinched shut when I bought the jeep (second hand) so I eliminated the the hanger ons.
The plot thickens...leaves me more confused.
I swear....this thing use to run great :}
post #7 of (permalink) Old 07-27-2002, 12:23 PM
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Re: Engine Missing under load

The verdict appears to be in! I "Think" I have it solved. The winner is: RRich!
I changed everything today! MCU bypass, stepper motor bench test, coil, coil wire, carb adjustments.....
The problem appears to be the darn platinum plugs. Put in a new set of cheapie autolite and she is back!
I can't believe a brand new set of plugs caused such a problem to my engine. More interesting, how do they work in some peoples Jeep.
Now I have to put all of this stuff back together!
Thanks for the help RRich and Jim Lou!
Keep your fingers crossed for me!
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