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-   -   Help! Engine backfiring when throttle is applied fast (http://www.wranglerforum.com/f210/help-engine-backfiring-when-throttle-is-applied-fast-71114.html)

chadsti 12-04-2010 11:17 PM

Help! Engine backfiring when throttle is applied fast
 
I have a 1997 TJ with a 2.5L. A few months ago i took it for a drive. it drove perfectly normal and fine. Then i parked it for about a week. Next time i tried to drive it it would not start. it would only turn over, so i had it towed to a shop in town. the Technician said the PCM was dead, so we replaced the PCM. Then he found the crankshaft sensor had failed. so that was replaced. the 2.5l would then start and idle but would stall as soon as any throttle was applied. he noticed the fuel pressure was low so he replaced the fuel pump. He noticed how ever there was a substance in the gas tank that appeared to be sugar. so he then cleaned the entire fuel system, replaced the injectors, spark plugs, along with the rotor cap, wires, and rotor button. the 2.5l would now idle great but would now backfire if the throttle was applied. he then replaced the throttle position sensor, and put some injector cleaner in the gas. this did help it a little bit, now if the throttle is applied slowly it will rev to red-line but if the throttle is quickly applied it will immediately buck and start to backfire. at this point i took the jeep home because he was stumped. At home i installed a new Map sensor, cleaned the IAT, replaced the air filter, and i seafoamed it. none of my efforts have had any positive or negative effects on the backfiring condition. i've noticed when it starts to backfire the revs will stay pretty steady in the 1,500 to 2,250 rpm range. No DTC's have been thrown. I am lost and i do not know what to do next. i feel like a fool for throwing so much money at this problem. If any one has an idea of what i should try next to fix it i will appreciate their help very much. I will list of every thing that has been replaced so far below. thank you!

Items replaced so far

-PCM
-Throttle position sensor
-Crankshaft position sensor
-Map Sensor
-Injectors
-Spark plugs
-spark plug wires
-rotor cap
-rotor button
-air filter
-Fuel pump
-Coil

jasonwrangler 12-04-2010 11:51 PM

wow such a big problem no check engine light?

brickboy3332 12-05-2010 12:05 AM

I had a somewhat similar problem and replaced the cam sensor or pickup underneath the distributor rotor and it fixed the problem. The crankshaft position sensor and cam sensor kinda work hand in hand with each other to send info to the computer. Its easy to change but costs probablly around 70 bucks jus a suggestion :\

chadsti 12-05-2010 12:26 AM

Quote:

Originally Posted by jasonwrangler (Post 908810)
wow such a big problem no check engine light?


I know. I even unplug a sensor to make sure the light still worked. lol

chadsti 12-05-2010 12:30 AM

Quote:

Originally Posted by brickboy3332 (Post 908831)
I had a somewhat similar problem and replaced the cam sensor or pickup underneath the distributor rotor and it fixed the problem. The crankshaft position sensor and cam sensor kinda work hand in hand with each other to send info to the computer. Its easy to change but costs probablly around 70 bucks jus a suggestion :\


thanks ill order one up and give it a try. i'm so deep in this now i cant turn back.

jpatches11 12-05-2010 03:26 AM

yea i had this same problem. i replaced the spark plug wires and it went away.however it came back :banghead: and now i am going to replace the distributor and spark plugs. that should hopefully be the problem. :eek::banghead:

s3nt3nc3d 12-05-2010 04:03 AM

I dont think your problem is your spark plugs...for one, check how your plugs are burning. Pull a couple and see if they're a nice tan color (normal). If they're white and look like chunks are missing out of them or like they're eaten away, you're running lean (and should replace your plugs amongst other things probably) and if they're black you're running rich (and should probably replace your plugs to start with a clean slate...while also fixing the condition causing it to run rich).

I'm not sure what the problem is... maybe O2 sensors reading incorrectly and causing the engine to adjust the fuel/air mixture improperly? On my old Dodge Dakota, a leaking intake gasket would cause backfiring.

One mistake I think some of you guys are making is you're throwing parts at it left and right. While yes, you may eventually figure out the cause, you're shelling out a lot of money in the process. Try to work on eliminating obviously dumb ideas...read up on other stuff to see if it would even cause that problem...and if you don't know, take it to a mechanic and make sure they fix it right the first time (or only charge you for what the actual cause of the problem is...so they're not just throwing parts at it either). To the original poster...that last part really applies to you. It sounds like your shop is throwing parts at it...and instead of saying, 'We screwed up and that didn't fix it afterall'...they're finding ways to justify you spending money on the unnecessary parts. IMO, you should take it back to them and demand they fix it right like you've already paid them...too much...to do or you'll take your business elsewhere as well as submit a complaint with the Better Business Bureau and if necessary, write a letter to the editor of the local newspaper about how they're taking all your money and not fixing the problem you're paying them to fix. Often times, that will make a local business take action...negative press = biiiiiig trouble for small businesses. They can't just shrug it off as a few customers lost.

rrich 12-05-2010 05:16 AM

It could be as simple as the spark plugs - s3's comments are right - but they don't have to have physical signs to still be bad or wrong.

But -- If the wrong plugs are in it you cannot overcome it by changing other parts.

Pull a plug, compare the brand and number with the underhood sticker - its on the firewall left side - are they exactly the same brand and number?

If not, put in the correct ones. Ignore what others say to use - use what the engine was designed to use! IT'S ON THE UNDERHOOD LABEL!

chadsti 12-05-2010 08:21 AM

Quote:

Originally Posted by s3nt3nc3d (Post 908929)
One mistake I think some of you guys are making is you're throwing parts at it left and right. While yes, you may eventually figure out the cause, you're shelling out a lot of money in the process. Try to work on eliminating obviously dumb ideas...read up on other stuff to see if it would even cause that problem...and if you don't know, take it to a mechanic and make sure they fix it right the first time (or only charge you for what the actual cause of the problem is...so they're not just throwing parts at it either). To the original poster...that last part really applies to you. It sounds like your shop is throwing parts at it...and instead of saying, 'We screwed up and that didn't fix it afterall'...they're finding ways to justify you spending money on the unnecessary parts. IMO, you should take it back to them and demand they fix it right like you've already paid them...too much...to do or you'll take your business elsewhere as well as submit a complaint with the Better Business Bureau and if necessary, write a letter to the editor of the local newspaper about how they're taking all your money and not fixing the problem you're paying them to fix. Often times, that will make a local business take action...negative press = biiiiiig trouble for small businesses. They can't just shrug it off as a few customers lost.

i thought he was doing this too.he gave me all the old sensors so i went on motor all data, and looked up the specs and testing procedure for every sensor that failed to meet spec. they had problems. hard part is all there problems never followed a pattern. one was shorted to power. the injectors where indeed cloged, the tps read out of range. the fuel pump was slow. :banghead:
when he had it the condition would improve a little after the sensor was change, tho it is still not driveable in realworld conditions. I put the new MAP on just because i had received two for another jeep i had. i figured it couldnt hurt. However he is telling me know if i run it it will go away on its own, and ya he just doesnt wanna lok at it any more all that it.

chadsti 12-05-2010 12:34 PM

this morning I went to the parts store and picked up a distributor pickup. it was only 20 bucks so i figured i would try it. It did not help.

i had a new upstream o2s from another jeep. i also installed that to see if that would help, it didn't also.

i plugged my code reader in to see if there was any pending codes. there was one it was p0500. Vehicle Speed Sensor Malfunction. So i looked at mine, the shaft that goes into the transfer case wouldn't spin easily. so i replaced it with a new Vehicle speed sensor. this did not help the backfiring issue however.


I took a video with my phone. this is exactly what it is doing.

http://www.youtube.com/watch?v=3nzXnDXgU4U

1st time i slowly push the gas to WOT

2nd time i push the gas right to WOT

3rd time i slowly push the has to about half then i quickly push it the rest of the way to WOT

4th time i slowly push the gas to WOT again

mholloway1809 12-05-2010 12:59 PM

Its sounds to me like the timing belt may be worn or have missing teeth, Pull the cover and check the condition of the belt and for residue on the cam position sensor.

Second, its not unusual for mechanics to keep faulty sensors from other jobs laying around to cover there ass when they start swapping parts because they cant diagnose the problem.

I ran my own repair shop for 10 years and i never charged a customer if i swapped a part and it didnt fix the problem, to have that many sensors go bad at once ?
Aint no way, get your money back, sue if you have to. PUT EM OUT OF BUSINESS !

rrich 12-05-2010 01:14 PM

I'll ask one more time because it's exhibiting the typical common symptoms -

Are the spark plugs the same ones the underhood label calls out?

Not what the parts idiot says works better, not what your neighbor says, not advertising hype says, not what fits a Yugo, not Bosch - but the EXACT SAME ONES THE FACTORY SUGGESTS?

JD Adams 12-05-2010 01:21 PM

The backfiring is due to excessive raw fuel being in the exhaust tubes; the cats will ignite it when conditions are right and there is oxygen to burn. On a PCM-controlled engine, this should not happen.

If someone did sugar the fuel tank, something in the fuel system may have been damaged and overlooked. I'd take it to another mechanic who specializes in Jeeps and get another opinion. It could be something as simple as a damaged fuel pressure regulator, an emission-related sensor or even a simple intake air leak. Hooking up a code scanner and comparing o2 sensor voltages should confirm fuel system problems, while other simple tests will pinpoint any air leaks if present.

jasonwrangler 12-05-2010 01:23 PM

Quote:

Originally Posted by mholloway1809 (Post 909278)
Its sounds to me like the timing belt may be worn or have missing teeth, Pull the cover and check the condition of the belt and for residue on the cam position sensor.

Second, its not unusual for mechanics to keep faulty sensors from other jobs laying around to cover there ass when they start swapping parts because they cant diagnose the problem.

I ran my own repair shop for 10 years and i never charged a customer if i swapped a part and it didnt fix the problem, to have that many sensors go bad at once ?
Aint no way, get your money back, sue if you have to. PUT EM OUT OF BUSINESS !

the 2.5 has no timing belt.:rofl:

jasonwrangler 12-05-2010 01:30 PM

did you check if the PCM is the right one? just a thought. seem like the Fuel Injection map is totally off and the rev limiter is wrong too.

rrich 12-05-2010 06:44 PM

Was OK, parked a week - now the PCM is the wrong one? Santa's elves needed the PCM but left the wrong one it it's place?


OP said """""it will immediately buck and start to backfire"""""

How will backfiring in the exhaust make it buck?

Backfiring in the intake will.

jasonwrangler 12-05-2010 07:19 PM

Quote:

Originally Posted by rrich (Post 909743)
Was OK, parked a week - now the PCM is the wrong one? Santa's elves needed the PCM but left the wrong one it it's place?


OP said """""it will immediately buck and start to backfire"""""

How will backfiring in the exhaust make it buck?

Backfiring in the intake will.

he replaced PCM once.

chadsti 12-05-2010 07:42 PM

Quote:

Originally Posted by jasonwrangler (Post 909318)
did you check if the PCM is the right one? just a thought. seem like the Fuel Injection map is totally off and the rev limiter is wrong too.


Interesting story there! i pulled one out out of another 1997 wrangler se that was exactly the same as mine in every way. the mechanic told me the one i had for an auto and the jeep dealer could not program. so he got me one off ebay. i do know he took it to a jeep dealer and they programed it for him. he had them bill me.

chadsti 12-05-2010 07:44 PM

Quote:

Originally Posted by rrich (Post 909743)
Was OK, parked a week - now the PCM is the wrong one? Santa's elves needed the PCM but left the wrong one it it's place?


it was replaced. I do know for a fact in the week i parked it it failed some how. my code reader would not read the old PCM at all, and the jeep would not start.

jasonwrangler 12-05-2010 08:55 PM

Quote:

Originally Posted by chadsti (Post 909822)
it was replaced. I do know for a fact in the week i parked it it failed some how. my code reader would not read the old PCM at all, and the jeep would not start.

maybe you can try swap it back in see if it works
Ive never hear PCM failures unless you burn it up.

chadsti 12-05-2010 09:04 PM

Quote:

Originally Posted by jasonwrangler (Post 909944)
maybe you can try swap it back in see if it works
Ive never hear PCM failures unless you burn it up.


Ill have to see if the mechanic still has it, go figure its the only thing i didn't get back. Maybe ill take it out and bring to to a jeep dealer to see if they can confirm the fitment.

chadsti 12-05-2010 10:05 PM

Quote:

Originally Posted by rrich (Post 909294)
I'll ask one more time because it's exhibiting the typical common symptoms -

Are the spark plugs the same ones the underhood label calls out?

Not what the parts idiot says works better, not what your neighbor says, not advertising hype says, not what fits a Yugo, not Bosch - but the EXACT SAME ONES THE FACTORY SUGGESTS?


im not sure. i know i did replace them 3500 miles ago. it ran fine till the day it wouldn't start. then once the pcm was replaced and it would start. it would do it with those plugs. the mechanic put a set Bosch plugs in. then when i got it back i tried a set of those e3 plugs. ill take a look and see what jeep recommends and try them. i hope its just my plugs.

rrich 12-06-2010 12:26 AM

Bosch? Do a search on here - there are many many threads about how Bosch plugs do that exact symptom!

Or better yet, just put in the right ones.


Please, Let us know how it turns out.

chadsti 12-06-2010 12:52 PM

I was wrong. the Bosch's where in when the PCM died. the mecanic put in a set of motorcraft, then i tried a set of those e3 plugs. i ordered up a set of oem champion's. ill give them a try

rrich 12-06-2010 01:16 PM

Sounds like most everything is new already.

Hopefully using the plugs the engineers designed the head around will do the trick. I have no experience with the E3's, but from I've read they are most likely in the category of "advertising hype."

If it doesn't, try finding someone that has and understands how to use an ignition scope. A performance dyno shop probably has technicians that can.

A good tech with a scope and a bottle of propane should be able to find the trouble in just minutes. Computer scanners won't do much to help.

Sounds to me the required voltage to fire the plugs is more than the ignition can provide. But that doesn't mean the coil is bad. We'll see.

Let us know!

chadsti 12-06-2010 01:47 PM

I hope they do the trick too. i am in my final year at Vermont Technical College. they have a automotive technology program here. i am going to meet with the head of the program this afternoon to see if he can take a look at it with the schools scopes. I will let every one know what he thinks is going on with it.

I run e3's in my subaru. they made a big difference with engine knocking, when i am forced to get sub-par quality fuel. But the performance mods i have in that car is a little different then my stock jeep.

rrich 12-08-2010 10:18 PM

Well?

Did you put the right plugs in it?

chadsti 12-09-2010 11:25 PM

I put the oem plugs in. They had no positive or negative effect on the condition. I now think it is either the distributor is out of sync a tiny bit or the pcm is programed wrong.

jeepjones 12-10-2010 04:46 AM

I'm sorry that your mechanic has taken you for a ride, I hope you find your problem soon. Have you tested the ignition coil? When a coil is going bad it will idle ok but when a load is put on it it will fire intermittently.

chadsti 12-10-2010 11:20 AM

the mechanic put a new one on it. I just tested it with a multimeter and it reads in spec. thanks for your input tho. :-)


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