• RME Gear & Apparel

91 xj poor performance and backfiring...

gijohn40

gijohn40

too poor to wheel... :(
Location
Layton, Utah
My son has done something to his 91 xj... it has very little power and kind of backfires when you mash down on the skinny pedal... when I had this xj a year ago it had plenty of power and could hold the cruise control at 75-80 without any problems on hills.... now a slight bump and the damn thing almost stops...

I replaced the cap and rotor cause they both were shot along with new sparkplugs (old ones looked good still) and even ran prem gas with some gummout injector cleaner... and even with the air cleaner off this thing is gutless... I checked all vacuum lines and all the spark plug wires and everything lookes great but still the problem exists...

So any suggestions from you brainiac would be very much appreciated!!!
John
 
Herzog

Herzog

somewhat damaged
Admin
Location
Wydaho
I'll suggest Throttle Position Sensor as well. There are ways to test it before you drop money on a new one. If you have access to a chilton or haynes manual, they usually explain how to test them with a multimeter.
 
DOSS

DOSS

Poker of the Hornets Nest
Location
Suncrest
You may also want to check you CPS (crank Position Sensor) they are notorious for going out on the I6....
 
gijohn40

gijohn40

too poor to wheel... :(
Location
Layton, Utah
ok I drove the xj around today for about 300 miles... from a standing still I floor it it stumbles and backfires up until the rpms hit about 3600 at which point the xj takes off... if I keep it above the 3600 rpms it has plenty of power and drives nicely...

Also once in a while when at low idle it will just die and not restart at all.. I found that if I pull the fuel supply hose it will start back up right away(?)...

I pulled both the fuel supply and return hoses and got sprayed by one and nothing on the other... so that is a good sign... I also unhooked the tps and isa connectors and looked them over and then put them back on.

Guess tomorrow after work I have some testing to do with the manual!

Thanks for all and any help you have in solving this trouble...
John
 
gijohn40

gijohn40

too poor to wheel... :(
Location
Layton, Utah
read the codes...

I just read the codes that were in the ecu memory and its telling me the oxygen sensor is reading lean and needs to be replaced.... would this cause what I have been experiencing?
 
DOSS

DOSS

Poker of the Hornets Nest
Location
Suncrest
your o2 sensor could cause it by it reading lean it is probobly putting more fuel in and flooding the engine...
Then again it could be something else out causing it to read lean.. replace it.. if it fixes it you are good to go if not return it and move on :)
 
DaveB

DaveB

Long Jeep Fan
Location
Holladay, Utah
If you want to come by the house I have some spare sensors (o2, map, temp, intake air temp and tps) and a spare computer you can try. I also have a fuel rail pressure gauge.
 
gijohn40

gijohn40

too poor to wheel... :(
Location
Layton, Utah
ok explain this...

when the engine is really choking and backfiring and then it dies and I can't get it restarted for abit... unless I relieve the pressure in the fuel rail at which time I can start it right back up...

When I check the engine codes the ecu tells me that the o2 sensor is running lean... I check the sensor and found that the heater elements are open so I bought a new sensor just for good measures...

This is really starting to get to me....
John
 
P

PREMIER.MOTORS

Member
Check to make sure your tank is venting properly. If it isn't venting it will create a vacuum and then when you release the pressure at the fuel rail you are actually getting rid of the vacuum pressure and allowing fuel back to your engine.
 
gijohn40

gijohn40

too poor to wheel... :(
Location
Layton, Utah
And The Answer Is....

I decided that since the thing was backfiring that I would check to see if the timing chain might be loose or slipped a tooth and guess what,,, there was almost an inch of play in it,,, so $50 later I have new gears, and chain on and I'm soaking the cover in dawn dish soap when it hits me that I didn't get a gasket set!

And to top that off I cut my finger pretty bad scraping the old gasket off...

So final testing will have to wait until morning....
 
gijohn40

gijohn40

too poor to wheel... :(
Location
Layton, Utah
still have some troubles...

ok I took out the fuel pump and looked at it... its producing about 30 psi of pressure... My manual says the fuel rail should have between 30-50 psi so I'm boarderline on it...

I did also find that the vacuum line going from the manifold alone firewall where it t's and goes into the heating/ac switch and then over to the cruise control then on to a vacuum reseivor up under the pass side front bumper had a few broken parts in it so I replaced it all...

I checked the CPS and its ok... I checked the tps and according to my manual once again it says the reading should be greater then 200 millivolts fully closed and a little less then 4.8 volts fully open... I got 600 millvolts closed and 3.8 volts fully open... is this within specs?

I also checked the coolant temp sensor and the manual says it should read about 7000 ohms cold and less then 1000 ohms hot.. I got 17k cold and 1200 hot... again is this in spec? or should I replace?

With replacing the vacuum hose and the timing belt and o2 sensor I have got the miss firing and stumbling down to between 1500 rpm and 2000 rpm... but I still have a hugh lack of power thru out the entire range.... HELP!!!
 
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