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should i just get a new motor???

JeepZiggy91

NAXJA Forum User
Location
bullhead city
91 4.0 aw4 242

for about a month now my jeep has been running like $hit, that is the best way i can explain it. huge loss in horsepower. exhaust sounds like an automatic bb gun. occasional backfire. popping. can clearly tell that a cylinder is missing. it started doing this occasionaly and then would go back to running like a bat out of hell. now it does it all the time and occasionaly it runs fine. while im driving i can shut it off and let it coast for a few seconds, start it and it will run just fine for a few miles or while im driving i can put it in N and rev it up to 5000 and it will run fine for a few miles too. you cant tell/hear it when its in park and at idle or even when you rev it.

ive taiked to a few mechanics and they've all said something different so here is a list of the things ive done to try to correct:

plugs
wires
rotor
cap
oil change
cleaned air filter
tps
map
o2 sensor on down pipe (no cat. no second sensor)
fuel filter
fuel sending unit
cleaned throttle body
fuel pressure regulator

i also have oil that comes out of my air filter, (blow-by??)

im at the point that im either gonna put a new motor in it or buy a new xj and keep this one for parts. id really love it if i didnt have to do either so please help.

ZIGGY!!!1
 
How many miles? When was the last time it had a tune up?

When I picked up my 98 from the Stealership here last May it had 124000 miles on it and ran like garbage. On the way home, picked up plugs, wires, cap and rotor. World of difference.

Pull the plugs first. If you have a timing light, you can use it to find the cylinder that is mis-firing. First things first...
 
You need to adopt a "testing" methodology, rather than installing parts and hoping for the best.

How many miles on this engine? A good snapshot in time of the internal condition of your engine is a simple compression test. Takes less than an hour. The compression spec for the Jeep 4.0 is 120-150 psi, with no more than a 30 psi variation. Inspect your spark plugs. Closely. The way they are burning can also tell a lot about what is going on internally.

Any check engine light? I would still check the computer to see if there are any "stored" codes which can yield some clues.

Have you actually tested fuel pressure with a gauge? It is the only accurate way to see if fuel delivery can be ruled out. The backfiring and popping can be indicative of either a lean mixture (backfiring in intake) or a rich mixture (backfiring in exhaust).

Most every engine management sensor in your 91 can be tested with a meter and a manual with the procedure. Never a bad idea to verify the operation of the coolant temp sensor, camshaft position sensor, etc......

What have you done to try to manage the oil in your air filter? When the CCV system gets clogged up, it can send oil into the filter when it is NOT true engine blowby. You need to know which your engine is suffering from.... real blowby OR a failing CCV system. I've seen people sell their XJs for a song and the new owner installs about $30 of parts and the oil problem is completely gone.
 
Last edited:
CPS
 
"I've seen people sell their XJs for a song and the new owner installs about $30 of parts and the oil problem is completely gone."

Exactly. That is what I did last May. A few minor cosmetic things to repair and a tune up.
 
this all started the day it hit 200,000. its at 201,000 now.

no check engine light, bulb is burnt out.

how do i do a compression test?

i have not had the fuel pressure tested, it just never came to mind...

last tune up was at 198,000

i havent done anything to stop the oil i had no idea where to even begin with that...ccv?
 

answers like this is what i was going off of and i just trusted the person's judgment and its still not fixed. im not to mechanicly savey so i need a little more explination but thanks for taking the time to type 3 letters.
 
no work to the head at all. the last thing i did before this started was put in a different gas tank which i cleaned before putting in.
 
Ok. I had a guy come to me with the same problem a while back and it turned out to be rocker arms not tightened correctly. Might be worth checking into...
 
this all started the day it hit 200,000. its at 201,000 now.

no check engine light, bulb is burnt out.

how do i do a compression test?

i have not had the fuel pressure tested, it just never came to mind...

last tune up was at 198,000

i havent done anything to stop the oil i had no idea where to even begin with that...ccv?

*Search this forum, other XJ forums including Jeep Forum and the web for lots of information about managing the oil in the air filter condition. A few parts and a couple hours of your time and you might be surprised at the results.

*Search also for how to run a compression test and how to test your fuel pressure.

*You may have a check engine light and you don't even know it because of the burned out bulb. Pull your codes with a code reader, a scan tool, or even the key-on, key off technique and see what stored codes are there that can really help you isolate a problem!! This is job #1! Below is a link to an article that shows you how to do this without a code reader, by just using a sequence of key on, key off steps. This should work for your 91.

http://www.go.jeep-xj.info/HowtoECUcodes.htm
 
I bet the CEL code will point in the direction of multiple miss-fires. Replace the distributor. The shaft/bushings have probably worn too much.
 
answers like this is what i was going off of and i just trusted the person's judgment and its still not fixed. im not to mechanicly savey so i need a little more explination but thanks for taking the time to type 3 letters.
sorry I left out the question mark behind it. I was asking if you had replaced it? crank position sensor. NOW stop being an a$$wad. I typed CPS cause everyone know what it means.
 
Ok, in my experience mechanical issues with an engine either exist, or not. Some rare exceptions to that rule, but not many.

An engine that runs shitty AND like a bat out of hell generally indicates something other than a mechanical issue.

First, renew all of the grounds. Have your battery load tested, clean the cables--both ends. Do a voltage drop test on the cables. If not already done, replace the POS braided ground strap that runs from the head to the firewall with a 4 or 2 gauge cable.

Bypass the fuel pump ballast resistor. TEST the fuel pressure, duct tape the gauge to the windshield so you can observe fuel pressure when the engine craps out. TEST the primary and secondary side ohms of the coil.

The oil in the air filter is more likely to be a clogged CCV system rather than blow-by.
 
The oil in the air filter is a seperate problem from the running problem. I hate spending money on parts hoping it fixes the problem. I'm old school. I diagnose problems.

The first thing I would do is what all seasoned mechanics tend to do. I would pull the spark plugs and take a look at them. If you don't know how to read them do a web search or better yet host the pix on photobucket or the like and link it here so one of us can give you an opinion. Plugs are at the heart of the engine and can tell you more than almost anything else, including a computer.

While the plugs are out, I would do a simple compression test. You can buy a gauge at Harbor Freight for around $20. It will tell you the condition of your rings and valves. I think Autozone also has them in their loaner tool program. Again do a web search for how to's and what the results represent. It will tell you if your engine is getting old and worn out and if it is on its last legs.

Personally, my bet is on clogged injectors based upon all you have said. I would run a can of BG44K through the fuel. Accept no substitutes. It will run you around $20 a can. Most NAPA stores have it. Checker/Oriely's/Autozone etc rarely sell it. I run a can every 10k miles in every rig I own. It is the closest thing to miracle in a can you will find.

The last thing I would check is to hook up a vacuum gauge to the intake. It should come with instructions. My second guess is a clogged CAT. Running crappy sometimes, wimpy sounding exhaust, and occasional popping through the "carb" are common symptoms. In that case when it is running crappy the vacuum will be low. You can run the hose out the hood and put it where you can see it when you are driving if necessary.
 
Plugs look OK. There is some buildup but not enough to cause what you describe. Any results on a compression test?
 
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