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Hard start/ backfire

dspilot

NAXJA Forum User
Location
Hickory Flat, MS
Several months ago, my XJ began to get hard to start. As time went on, it became more difficult. Eventually it got to the point that a cold start was sluggish, and a hot start (after pumping gas or something), was down right iffy. It would always start, but I could smell raw fuel afterwards. Now if I let it sit idle for a while and it gets hot, it will die off and be impossible to start for thirty minutes to an hour (after it cools down a bit). A lot of times after starting it warm, it will run really rough and backfire through the throttle body.

Things I have done recently: fuel injector cleaner, plugs/wires/ dist. cap/ rotor button, cleaned throttle body. Fuel pump is about three years old, and the sensor under the distributor cap (can't remember the name) was replaced about two years ago.

Reading in other posts with similar problems, I am thinking maybe a sensor on the TB (MAP, TPS), or possibly an oxygen sensor. Also, thinking maybe an issue with timing, or exhaust, or possibly ignitor coil.

Any insight or thoughts would be greatly appreciated.

96 XJ, 4.0, 4wd
 
I am by no means an expert but I've read from a few other posts that backfiring during startup could be a intake manifold or exhaust manifold seal problem.....something that's fairly common for our XJ's....you might inspect the system for any cracks or perhaps squirt some starter fluid around the manifold while its idling to see if the engine rpms change......increase revs=leak somewhere

Again, I'm no expert but just thought I would add my $.02
Good luck

Also, has your fuel mileage dropped off recently? If so, it would make sense if you had a lean condition causing the computer to add more fuel...could also be the O2 sensor you mentioned if your mileage has dropped in conjuction with this new problem
 
Sounds like a failing CPS/CKP, test the crank sensor.
 
so, today i replaced the MAP sensor, coolant temp. sensor, and upstream O2 sensor......no change. I drove it to AZ to get codes pulled again, and came up with P0108, P0171, and P0113. these were pulled before the O2 sensor was replaced, and after the MAP and CTS were replaced. How would I go about checking the CPS, and would it still be bad although no OBDI codes point to it? Thanks guys. this is getteing rather frustrating.

Also, the P113 code indicates intake air temp. sensor....would it being bad cause these symptoms? It is only 13 bucks to replace, they just didn't have it in stock while I was there.

Finally, would replacing the downstream 02 sensor make a difference? I unplugged it, and got no change from the vehicle.

ETA: I did spray starter fluid all over the intake manifold, and found no leaks.
 
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Replacing the downstream o2 sensor will not make any difference. It's only job is to monitor the performance of the catalytic converter. It has no effect on your air/fuel mixture, that's what the upstream o2 sensor is for.
 
You can use an ohm meter to measure the resistance at the CPS/CKP, sorry, I don't have the 96 specs available, but do a search and you will find them.

It really sounds like a marginal CPS/CKP which is failing under thermal conditions. Get the engine warm so that it will fail, then pour cold water on the CPS/CKP and see if it will start.
 
Replacing the downstream o2 sensor will not make any difference. It's only job is to monitor the performance of the catalytic converter. It has no effect on your air/fuel mixture, that's what the upstream o2 sensor is for.


That's what I thought. Thanks for calrifying.

Joe: great idea. I will check it immediately. The water trick that is.
 
OK, water trick made no changes. It actually seems to be running worse now with the new 02 sensor by the way. I am really getting frustrated with this.. Could it be fuel injectors, or a problem with the cat. I was just going to take the cat off, and see if it made a difference, but it is welded on, and I am not quite wanting to cut it off yet.

Also, is there any chance that it could be the TPS?
 
I don't see the TPS causing no start condition.

Coil, CPS/CKP are two items very suspect in thermal failures.

Ok, back to basics. Put it in a no start condition, then check for spark.

Also, put a fuel pressure gauge on it and post back the psi.
 
replaced the coil tonight....no change. really starting to think it is a fuel issue. I talked to a mechanic that I really have a lot of respect for, and he seems to think its a fuel issue also.
 
Run some tests on it before you throw more parts at it. Get a hold of a mechanical fuel gauge and post the results here.

I'm thinking fuel issue or the early stages of a failing CPS (as mentioned above).
 
i tried to get a fuel guage from autozone, but they only have them for sale. I want to see if I can find one to borrow before I buy one. Seems a little silly, I guess, that I am willing to blindly throw three hundred dollars in parts at it, but am too cheap to spend forty bucks on a guage..
 
Spray ether (starting fluid) into the intake while cranking--if it fires it is a fuel problem.

If you determine it is a fuel problem, then you need to determine what kind of fuel problem--no pressure, low pressure, too much pressure, pump not working, pump not getting power, injectors not firing.
 
took it to the shop. pressure checked in at 20ish psi. all indidcations point to a fuel pump. the mechanic there agreed. My question-- is there a pressure regulator in the system that could be going bad, or is the pump the likely culprit. The current pump in the Jeep is about four years old with about 30k miles on it. Should it really be going bad already?

ETA: the haynes maual (sucks, but its what I have) shows a pressure regulator on the fuel rail, but it is not at all like the system in my Jeep. I think Haynes has just thrown a bunch of random pictures in with the manual. There is no pressure regulatior on my fuel rail.
 
took it to the shop. pressure checked in at 20ish psi. all indidcations point to a fuel pump. the mechanic there agreed. My question-- is there a pressure regulator in the system that could be going bad, or is the pump the likely culprit. The current pump in the Jeep is about four years old with about 30k miles on it. Should it really be going bad already?

ETA: the haynes maual (sucks, but its what I have) shows a pressure regulator on the fuel rail, but it is not at all like the system in my Jeep. I think Haynes has just thrown a bunch of random pictures in with the manual. There is no pressure regulatior on my fuel rail.

if not on the fuel rail, then the fuel pressure regulator is part of the fuel pump assembly. i belive the 96 is an odd year where there were changes made after mid-year. you need to find the build date to figure out what system you have.
 
Either the regulator is at the front of the fuel rail, with a vacuum line attached, or it is on top of the fuel pump assembly in the tank.

Your OBDII 96 would have the regulator in the tank, and your pressure should be 49 psi. You can try replacing the fuel filter and checking for any kinks in the fuel line before replacing the pump.

Normal fault for a pressure regulator is HIGH pressure.
 
its definitely in the tank then. i will be ordering another fuel pump it looks like. Any specififc brands to look for or avoid. the one in it now is a knockoff no name brand that I know of. I think I should at least go with something recommended here.
 
ETA: the haynes maual (sucks, but its what I have) shows a pressure regulator on the fuel rail, but it is not at all like the system in my Jeep. I think Haynes has just thrown a bunch of random pictures in with the manual. There is no pressure regulatior on my fuel rail.

Just FYI for everyone, the Haynes manual says it goes up to 2001, but it actually stops at '95. And now here's one of the key differences from 95 to 96, the fuel delivery system changed. Dunno about Chilton's book, but I'm 95% sure they're the same people :)
 
Just FYI for everyone, the Haynes manual says it goes up to 2001, but it actually stops at '95. And now here's one of the key differences from 95 to 96, the fuel delivery system changed. Dunno about Chilton's book, but I'm 95% sure they're the same people :)

Try and find genuine Chryco manuals on ebay/craigslist, stay away from the pirated crap. You can get LICENSED reprints and CDs from http://www.autobooksbishko.com/index.html

The problem with the Haynes and Chilton manuals is they try and cover too many years. The Haynes used to be very good in the 70s.
 
yeah, I have a Haynes bicycle repair book that pulls the same junk. they may go into great detail on cantilever style brakes, and hardly even mention anything else. I guess that is what was big when the book was published. They could do a much better job if they would break all of the info down into severeal books.
 
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