1991 4.0 Loss of acceleration / power - JeepForum.com
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post #1 of 21 Old 06-08-2012, 03:05 AM Thread Starter
MSUDawg
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1991 4.0 Loss of acceleration / power

1991 Jeep Wrangler- 6 cyl
- 4.0 fuel injection
- Jeep is OEM (tires, wheels....) No modifications
- Mileage 185,000 - New Engine 20,000
- The day before I noticed this, I got gas, so I thought it was bad gas.

Problem -
  • I find myself using higher RPMs before (1st through 3rd) shifting gears. If I shift when the light states or when I us to it feels like the motor is going to bog down (almost like I have a brake on or pulling a large load ).
  • Doesn't have the zip it us to have (one week ago). Tops out at 65 mph on hwy and if wind is blowing against me or going up hill I will loose 10 mph.
  • Down shifting doesn't really help much (5th to 4th to accelerate).
  • FYI - It sounds great and runs well until you put some stress on it (driving)
  • I believe I am not getting the volume of fuel needed when jeep is under stress. I know a fuel pump going out would cause this but is there anything else?????

What I have done so far -
  • Ran a tank of Sea Foam
  • Took it to Mech, who charged me $135 to tell me I have good pressure in the fuel line (not while driving just in garage) and replaced Distributor Cap / Rotor (not going back to him)
  • Changed Fuel Filter, test drive and though I had it fixed but the next day it was running sluggish again
  • Thought maybe trash in fuel, pulled fuel tank and replace fuel filter again but couldn't find any trash. Again test drive was good at first (I could burn rubber) but returned to sluggish ways. One side note for a little while you would feel a boost then back to sluggish, boost then sluggish all within a few seconds.
  • Mechanic said catalytic converter was ok

Replaced:
  • Spark Plugs
  • Air Filter
  • Wires
  • Distributor Cap
  • Rotor
  • Fuel Filter (2)

FINALLY I HAVE TO ADD THIS!
The day I noticed I had left my lights on and had to jump start my car. I crossed the wires and left it on for 15 seconds. Could this have fried a sensor or computer that would cause the sluggishness?

Current plan of attach - FEED BACK WELCOME!
  • Change computer (I have an extra)
  • Change Coil
  • Change O2 sensor
  • Change TPS
  • Change fuel pump

PLEASE HELP, Tired of being passed by mini vans!


Last edited by MSUDawg; 06-11-2012 at 08:12 AM. Reason: ease of reading
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post #2 of 21 Old 06-08-2012, 08:41 AM
Que89YJ
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2004 LJ Wrangler 
 
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Check your fault codes:
https://www.jeepforum.com/forum/f12/h...codes-1257145/

You should have 12, 33, and 55

Is there any difference cold/warm?
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post #3 of 21 Old 06-09-2012, 02:03 AM Thread Starter
MSUDawg
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Yes, only codes I get are 12, 33 and 55.

Cold or hot? It seems to have a little more power at first but quickly disapears!

?
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post #4 of 21 Old 06-09-2012, 05:13 AM Thread Starter
MSUDawg
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It is defiantly better when cold but still acts like it has bad gas. Or it surges for the first 1/2 miles.
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post #5 of 21 Old 06-09-2012, 10:05 AM
Que89YJ
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You need a meter. The temp sensor near the thermostat housing would be the thing to check.

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post #6 of 21 Old 06-09-2012, 10:07 AM
Que89YJ
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Also have you checked for a blocked or bad cat.

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post #7 of 21 Old 06-09-2012, 12:42 PM
50rck
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Que89YJ beat me to it. First thing I thought.

1991 YJ, rebuilt 4.0L in spring 2009 at 234,000 miles, no lift yet, slightly offset wheels, 235/75/15 tires for now, rag top, sold the hard top. Over 35,000 miles on the rebuild and counting.


2006 Jeep Liberty. Nothing special, all stock.
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post #8 of 21 Old 06-09-2012, 01:14 PM Thread Starter
MSUDawg
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Quote:
Originally Posted by Que89YJ View Post
You need a meter. The temp sensor near the thermostat housing would be the thing to check.
Do I really need to check it if it is showing up on the instrument cluster or is that another gauge?


Quote:
Originally Posted by Que89YJ View Post
Also have you checked for a blocked or bad cat.
Mechanic said catalytic converter was ok

Thinking what governs the amount of gas that the injectors spray?
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post #9 of 21 Old 06-09-2012, 06:05 PM
Que89YJ
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There are 2 different temp sensors. I would check the cat. I am not sure the best way to test it but the description is more like it is intermittent. Usually sensors just fail. The other thing is maybe arc over from a cracked coil.

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post #10 of 21 Old 06-11-2012, 08:12 AM Thread Starter
MSUDawg
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OK - I tried to test the Temp Senor, but couldn't quite figure out if the ohm reading I was getting was good or not so I just went out and bought one. Sadly it didn't fix the problem. So I have an spare temp sensor
I also change out ECM just to check it off, not help.

Here is a recap, when I start driving it, cold engine, it runs GREAT! as it heats up it runs more and more lean. I can feel it going from great to lean, because it surges, as if you have bad gas or running out of gas. I also tried to turn it off and wait a few minutes and see if the problem still there and it is. As long as the engine is hot it runs lean.

I have replaced, Spark Plugs, Air Filter, Wires, Distributor Cap, Rotor, Fuel Filter (2), Temp Sensor (not the one that controls the gauge) and ECM

Mechanic has checked fuel pressure (not under stress) and he said it was OK.
Mechanic has checked catalytic converter and he said it was OK. Not 100 sure if he knows what is doing because he didn't pressure test it, but I am taking his word for it. I also don't think it is because this problem came on over night and wasn't gradual. But I will take it to a muffler shop last.

Any other suggestions?
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post #11 of 21 Old 06-11-2012, 08:47 AM
pete1991YJ
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O2 sensor - the computer doesn't read it until the temp sensor tells it to - when the temp sensor tells the computer to read the O2, the computer then uses the O2 reading to set the air/fuel mix.
.

***--- Wrecking Crew member ---***
"I've prolly been around longer than you think"
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post #12 of 21 Old 06-11-2012, 09:16 AM
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Cold and warm is the key X100 with Pete! Good mornin' Pete! Check the O2 sensor and check the exhaust manifold for any cracks or loose bolts. The cold/warm was the key.
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post #13 of 21 Old 06-11-2012, 01:53 PM Thread Starter
MSUDawg
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Just replaced oxygen sensor. No change. Next idea ?
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post #14 of 21 Old 06-11-2012, 02:22 PM Thread Starter
MSUDawg
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Only codes are 12 55
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post #15 of 21 Old 06-11-2012, 07:10 PM
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Cracked exhaust manifold or loose bolts on it. Bad temp sensor near tstat.

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