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post #1 of (permalink) Old 09-24-2005, 10:31 AM
ralph77
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98 jeep wrangler 4.0 backfire

Hello. I just completed a engine swap on a 98 jeep wrangler. The engine that lkq sent me belonged to a cheroke, they said it would work, I had to swap all of the sensors off the old motor and put on new motor.also distributor....the new motor did not come with a fly wheel so i used the old one...I set both motors to tdc. marked the fly wheel and swaped. the crank has an extra hole beside the bolt holes where the flywheel bolts on..and both engines at tdc had the hole in the same spot...so hopefully i did good there....i put engine in....while fasting the torq converter bolts i locked the engine up. I forgot to tighten the top bell hosing bolts ..once i did engine freed up .........finished installing engine..... engine would not start at first but then it fired, idled wonderful,,until you gave it gas.....major backfires....everything is in time..nothing is crossed..i am not 180 out...swaped the cps sensor..did not change backfire....bought new coil, plugs,cap and still backfires..firing order is correct...now for my questions..could i have damaged the timing ring on the flywheel and the cps cant pick up the bad spots...but remember it idlesfine but hard to start...is the computer bad...i Had to replace a fried ground that attaches to the passenger fender well ...the wire attaches some where on the block??? does the intake and exhaust on a cheroke match that of a wrangler???? Help thanks
Ralph
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post #2 of (permalink) Old 09-24-2005, 11:04 AM
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Re: 98 jeep wrangler 4.0 backfire

Check the MAP sensor...
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post #3 of (permalink) Old 09-24-2005, 12:06 PM
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Re: 98 jeep wrangler 4.0 backfire

i bought a new map....and it still backfires....the exhaust manifold is glowing like a neon sign in vegas. engine is still hard to start....help....thanks ralf
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post #4 of (permalink) Old 09-24-2005, 12:26 PM
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Re: 98 jeep wrangler 4.0 backfire

bought a new map....and it still backfires....the exhaust manifold is glowing like a neon sign in vegas. engine is still hard to start....help....thanks ralf

i also ran a compression check and all 6 were at 150...good compression....why would the exhaust manifold glow like that my cat is not stoped up....
post #5 of (permalink) Old 09-24-2005, 02:15 PM
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Re: 98 jeep wrangler 4.0 backfire

What was the old motor and why did you swap it out?

Could you have an exhaust system problem behind the converter?

Sorry, I don't know very much about the FI engines.
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post #6 of (permalink) Old 09-24-2005, 04:07 PM
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Re: 98 jeep wrangler 4.0 backfire

thanks for the reply jim
the old motor is a 4.0

someone ran the old engine too hot and warped and cracked the head.with the head off the engine i saw damage to the lower end as well...instead of building the engine,i found a used 4.0 for $650.00 with 70,000 miles.so i thought that was the cheapest route.it proably would of been cheaper to just build the darn thing to start with...i was just tring to save on the headaches......what made the engine run hot to start with was that the radiator plug was broke.

i took the crossmember off so i could beat the heck out of the converter and that did not fix the problem...i also disconected the exhaust manifold...and the engine still backfired when giving it gas.


the distributor has a cam sensor built in it....could that be the problem..but if it was i would guess that the thing wouldent crank in the first place..i dont know....any help and i would appreciate it....im still thinking computer

the jeep is hard to start but when it does it idles great except when you give it gas...theres is plenty of pressure in the fuel line...could the glowing exhaust manifold and backfire be a result from a rich mixture....i dont know....help please......ralph77
post #7 of (permalink) Old 09-24-2005, 05:13 PM
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Re: 98 jeep wrangler 4.0 backfire

What TPS is in the engine? the original one from your first engine? Adjust the TPS I assume you have the auto, so an auto TPS should be used and it should be adjusted with the auto procedure .

Is the cap still OK? No dirt to cause crossfiring?

1 Thing that sometimes helps is disconnecting the battery for an hour or so, to make the ECM reset.

Oh, and the flywheel can't be mounted wrong. The only mistake possible is to leave a flywheel on a newer engine and use it in an older system with different teeth. That won't work, but you told you switched the original flywheel to the replacement engine...
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post #8 of (permalink) Old 09-24-2005, 07:08 PM
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Re: 98 jeep wrangler 4.0 backfire

I think your problem is with the engine control computer, but I really can't help beyoud that. A cam out of time could do that too, but that's not likely to be the problem if you didn't fool with the timing chain.
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post #9 of (permalink) Old 09-24-2005, 09:00 PM
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Re: 98 jeep wrangler 4.0 backfire

The Jeep's running lean, the timing's advanced, or it's got a clogged exhaust system... or a combination of the three.
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post #10 of (permalink) Old 09-25-2005, 09:17 PM
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Re: 98 jeep wrangler 4.0 backfire

I'd start simple.
One step at a time -

First check the timing at idle with a timing light. If it's close that's not it. If the flywheel trigger was very far off, it would be firing way off - the timing marks are still on 0 no matter which engine. Unless you also switched vibration dampers.

Second - to check if it's too lean - (hot manifold can get hot from being lean - it misfires, the unburned fuel burns in the exhaust) - do a searh here for "propane trick." Make the necessary mods to a propane valve - as original you won't get enough propane through the valve.
Idle - add propane to see if it runs much better. Propane will burn just like gasoline under these circumstances.

Usually too rich won't backfire or stumble - and you'd be seeing clouds of black smoke too if it was rich enough to backfire.

DO NOT try pouring a little gas down or a spraying in carb cleaner like Gumout, or ether - DON'T TRY IT - not with the backfiring - you could get badly hurt or burn the project down. Propane is relatively safe, but still -- keep your face away!

Third - drop the exhaust down - disconnect the headpipe at the manifold. After you run it, put something up in the manifold ends to keep cold air off the valves to keep the hot valves from bending. Wet rags works fine.

I'd not blame the electronics - yet. The basics of compression, spark, mixture are still valid no matter what electronics are used.

Since it runs it's not likely the computer, possible, but doubtful.
Since it runs, it cannot be 180 out, it'd bacfire but cannot actually run like that. But it still could have a couple of plug wires mixed up -- sometimes when we check we do the same mistake over and over.

Let us know how it fares.
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