1990 Jeep 4.0 Straight Six Cuts Out when you give it the gas - JeepForum.com

 
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post #1 of 13 Old 04-07-2012, 11:03 AM Thread Starter
Jeep334
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1990 FSJ Cherokee 
 
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1990 Jeep 4.0 Straight Six Cuts Out when you give it the gas

Hi all,

I just bought the Jeep last night, the battery was dead so I put in a new battery and it started and idles great, but we you give it the gas it cuts out and backfires... when driving if you feather the throttle it will get up to about 30 mph but thats it if you try to give it more gas to accelerate it just spitter spatters and cuts right out.

I put a new battery, fuel filter, new gas with sea foam, new air filter (after market cone style) in last night.

Any helpful tips to figuring this out would be great... Thanks!

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post #2 of 13 Old 04-07-2012, 11:07 AM
Saudade
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You need to check the fuel pressure with a pressure gauge on the test port.
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post #3 of 13 Old 04-07-2012, 11:12 AM
Z2_2
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I am having pretty much the same problem. I bought my daughter a 1993, 4.0 h.o. It runs great until you kick it down to passing gear then it falls on its face. I have to pull over shut it off and then it will be fine. The fuel pump seems to pressure up ok, I havent done a pressure test, could it be a fuel pressure regulator? Any how didnt mean to step on your thread but we share a common problem and maybe we can get a common answer. Thanks in advance for any replies or suggestions.
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post #4 of 13 Old 04-07-2012, 11:25 AM
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by Saudade View Post
You need to check the fuel pressure with a pressure gauge on the test port.
This.

Both of you guys need to verify fuel pressure with a gauge. No shortcuts. A gauge is the only way to accurately verify fuel delivery.

Many parts stores will rent you a FP gauge inexpensively; call around.

For both the 90 and 94, you should have 31 psi at idle. The disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be darn sure that the vacuum line to the FPR is pulling good vacuum and is in good shape with no cracks, especially on the connection end. If that line has raw fuel in it, you have a ruptured fuel pressure regulator.

Along with fuel pressure, be sure you have solid tuneup hardware. Fresh Champion plugs gapped to .035, new plug wires, distributor cap, rotor and fuel filter. Importance of this stuff cannot be overstated.

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #5 of 13 Old 04-07-2012, 11:37 AM Thread Starter
Jeep334
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At a idle with the hood up, if I give it the gas by pulling the the throttle line down it almost sounds like it is over fueling and you can smell the gas like crazy, sometimes it will backfire out the intake? Sorry to add more to the list... and thanks for all the good tips. I can't wait to get this jeep on the trails!
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post #6 of 13 Old 04-07-2012, 05:21 PM
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by Jeep334 View Post
At a idle with the hood up, if I give it the gas by pulling the the throttle line down it almost sounds like it is over fueling and you can smell the gas like crazy, sometimes it will backfire out the intake? Sorry to add more to the list... and thanks for all the good tips. I can't wait to get this jeep on the trails!
Backfiring is often due to an "imbalance" in fuel air ratio.

And you smell fuel.

Get a fuel pressure gauge and start testing as I outlined. The observations are fine, but testing tells the story.

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #7 of 13 Old 04-07-2012, 08:03 PM Thread Starter
Jeep334
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Fuel psi is good... Do you think it could be the injectors? or a injector?
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post #8 of 13 Old 04-08-2012, 06:05 AM
cruiser54
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Quote:
Originally Posted by Jeep334 View Post
Fuel psi is good... Do you think it could be the injectors? or a injector?
Did you determine this with a gauge? If so, what was the reading?
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post #9 of 13 Old 04-08-2012, 03:17 PM
grainofsalt
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Is the alternator belt tight? How does your cap and rotor look? Have you checked the coil?
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post #10 of 13 Old 04-11-2012, 04:37 PM Thread Starter
Jeep334
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New Cap, Wires, Plugs, and the Coil seems to be working great (good spark) I just got a timing gun, I will check the timing and re-post later. Thanks for some more helpful Ideas!
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post #11 of 13 Old 04-11-2012, 05:33 PM
tjwalker
It's the crank sensor!
 
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Base timing is controlled by the computer. What were your fuel pressure numbers. Not trying to harp here, but this has to be solid.

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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post #12 of 13 Old 04-11-2012, 06:02 PM
sparklingpickle
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I just put a motor n my 89 out of 87 and the 89 cps was just a tad longer than 87 so I swapped them and it ran great... try cps
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post #13 of 13 Old 11-24-2015, 02:19 PM
DE4N
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Listen to TJWalker! Check your fuel pressure. I had a similar problem and just started throwing parts at it at first. New fuel filter, new rubber hoses on fuel line (they were cracked), new fuel pressure regulator. Then I read this post and put a fuel gauge on it. That led me to bad relay. TJ knows the ropes, if you are reading this down the road, as I did, check your fuel pressure first!

Every once in a while, even a blind squirrel finds a nut!
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