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Greg, Technician
Category: Jeep
Satisfied Customers: 6730
Experience:  maintain jeep wrangler's for a off road rental company,
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1990 jeep: blown..4.0.The engine starts and idles fine,timing..intake

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I have a 1990 jeep 4.0 with a blown engine.I replaced it with a 1995,4.0.The engine starts and idles fine,timing is on.Above idle the engine starts to run very rich,at 2000 rpm it's back firing in the exhaust,not thru the intake.I have removed the exhaust pipe after the cat.and checked it with a light it appears to be open.What's your take on this????HAL
Hi welcome to will answer your question the best I can,

Hal if you are misfiring though the exhaust then you don't have the timing spot on or you have a burnt exhaust valve .

Do a compression test then get back to me with the readings .

Customer: replied 4 years ago.

Greg,compression is 140,vaccum 14-15"@idol thru 1200rpm and then goes to pot with the backfiring,a double check of the timing(for the fifth time)with a timing light shows it to be right on,,,question can the crank sensor be installed backwards,or will it fit only one way?????Hal

Customer: replied 4 years ago.
Greg,if this was a carb.set up I'd sware the choke was stuck on.That's the way it behaves HAL
HAL i bet you have the firing order out, this is really easy to do it happens a lot with this engine . normally the 3 and 4 are crossed.??

Check from my diagram .

If this is okay please get back to me.

Kind regards Greg
Greg, Technician
Category: Jeep
Satisfied Customers: 6730
Experience: maintain jeep wrangler's for a off road rental company,
Greg and 4 other Jeep Specialists are ready to help you
Customer: replied 4 years ago.
Greg,I've bouble checked the firing order several times,IT'S RIGHT,,Question,,mabey the problem starts with the distributor,,,the distributor I'am using is a 1995 unit,,is it compatable with the 1990 electronics??????Hal
Customer: replied 4 years ago.
PLEASE answer my questions Hal
Hi Hal

If the distributor are the same with the same connection plug then they are ok ,

What spark plugs are you using Hal

Customer: replied 4 years ago.
Greg,The plugs that I'am using are for a 1995/4.0..I just went to NAPA and they tell me the distributots are NOT interchangeable,,therefore I've ordered a 1990 unit,should be in by Thursday,,will advise....Hal
Hal does your model have the ignition module under the coolant bottle

Looks like this one graphic

Customer: replied 4 years ago.
Greg,NO my ignition madule is under the coil...Hal
See if you can use one to try out, possible problem ?

Customer: replied 4 years ago.
Greg, I guess it's possible. I will have the new distributor tomorrow and we'll see.
Hi Hal .

You never did say what plugs you are using ?
autolite 985 ?or another make but let me know .
most plugs are great but if you have Bosch plugs get them replaced
as this plug will give you the problem you are having ?

Good let me know what happens .

Customer: replied 4 years ago.
Greg,I Changed out the distributor today,engine is runnng fine,,,Hal
Great new, thank you Hal .

regards Greg

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