4.0 sluggish acceleration and stalling

[Hog Head]

Jeep friends,
I've just finished my build of an 83 CJ7 with a 93 4.0 newly rebuilt engine, put in by previous owner. Engine had never been started when I bought it. With the help of this forum, finally got it running and am presently doing some test runs on the road and on the trails. It starts every time with no problem. At lower speeds seems a little jerky and on the road accelerates very slowly. And if I'm not careful, at most stop signs completely shuts down. Will start back up right away with no problem. It seems like fuel starvation, but I really need you guys expert help here.
Everything is new, in tank fuel pump, external fuel filter, every sensor except cam sensor, new coil, wires, plugs. I'm curious if maybe when I indexed the distributor, if I'm off maybe by one tooth. Would that cause the timing to be off enough for these symptoms? Or do you think it could be more of a fuel issue? Please let me know if you need more info, and thanks in advance for your help.


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[Lumpy Grits]

Do a compression and leak down test.
LG

 

[Hog Head]

Thanks LG, did the compression test sometime back when I had starting issues. I don't remember the numbers, but all were good.


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[Lumpy Grits]

Then look at the camshaft timing.
Is the carb'd or FI?
LG

 

[Hog Head]

FI. I have a new camshaft sensor but haven't put it on yet. Best I remember, I couldn't figure out how to take the distributor apart and install it.


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[Belizean]

I have a 96 Cherokee since new with 4.0 HO. Check the butterfly on the throttle body for any dirt or carbon. That can really throw things off even if just a slight buildup. Maybe it was not cleaned up before installation?

 

[Hog Head]

Thanks for all the replies guys. I'm gonna take a look at all the items you mentioned first thing tomorrow morning. I'll let you know what I find.


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[mtnwhlr]

Have you checked the fuel pressure at the fuel rail?

 

[Hog Head]

Checked the fuel pressure at the rail and here's the numbers.
Key on no start - 40 psi
Key and start - 30 psi
When accelerating throttle - advances to around 38 and back to 30. How do those pressures sound?


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[mtnwhlr]

Yea I don't think that's your problem. Is it kicking out any codes?

 

[Hog Head]

Yes sir kicked out the following codes
13
23
35
73
75
Just replaced the map sensor today.


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[mtnwhlr]

Have looked into what all those codes are?

 

[Hog Head]

Yes sir
13-map sensor (got this code after putting new one on)
23-intake air temp ckt
35-cooling fan relay
73-can't find anything on this code
75-can't find anything on this one either
I did find a friend of a friend who's a retired jeep dealership mechanic and has agreed to help me. Said he had an old obd1 scan tool. I'm guessing it would be more accurate than my shade tree mechanic malfunction light. I'm gonna get with him next week and take a look.


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[mtnwhlr]

I can't find anything on those two codes either, pretty strange. Wish you luck.

 

[Hog Head]

Thank you mtwnwlr and all my jeep CJ forum friends for your advice. Hopefully I'll have an answer after visiting with my new jeep mech friend. I'll keep you posted.


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[Joebronco]

Checked the fuel pressure at the rail and here's the numbers.
Key on no start - 40 psi
Key and start - 30 psi
When accelerating throttle - advances to around 38 and back to 30. How do those pressures sound?


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Fuel delivery. I had the same issue with mine until recently. Replace your fuel filter. Even if you just did it.

When you went to the 4.0 motor did you go with an in-line fuel pump or in tank?


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[Hog Head]

I used an in tank pump and the filter that came with it. Mounted the filter in the engine compartment on the wheel well.

My jeep mech friend said pressures were ok. We did change the size of the vacuum hose on the rear crankcase vent. Went to a very tiny hose. He had a Chrysler diagnostic tester that would plug into obd1 connection. Told me a lot about the engine. But anyway, with the large hose, it was pulling way to much vacuum and not allowing the idle air control to work properly. That cured my stalling issue and he said looking at the age of the injectors, could run some cleaner through them and that should clear up the sluggish acceleration issue. It's been running much better since he checked it over for me. With 2.73 gears and T-18 4 speed it should run 70 mph. I did get it up to 60 last night. Cleaning the injectors I'm hoping is the last thing for the engine. Thanks again to all you guys for the much needed help.


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[Joebronco]

Not necessarily pressure but flow. The CFM gets choked down in the filter. I had 80psi to the rail (knocked down to 44psi at the rail) but I had restricted flow due to the clogged filter.

All that stuff the low pressure filter picked up was nothing compared to the high pressure filter.


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[Hog Head]

I'm not sure how to measure flow (cfm). But the tank is new, pump is new and filter is new. I've only filled the tank two times. Seems like there'd have to be an awful lot of trash in the 30 gallons of fuel I put in it to clog a filter that quickly.


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[Lumpy Grits]

If you run a 3/8" hard line from the tank, to the engine you will have the CFM needed.
The OEM factory line is only 5/16".
LG