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backfire out throttle body/power loss

jeannies_driver

NAXJA Forum User
alright i searched and racked my brain and still nothing...

i have an 89 AW4 4.0
felt kinda funny yesterday and this morning driving to work, had a " new feel " that just wasn't right. on the way home i leave a red light and it pop's back through the TB. highway running it was ok but when the TC locked up in 4th gear it would just chug a bit then down shift and run fine and would then up shift and do it all over again.

it idle's fine, no miss or rough anything. but if i give it 1/2 or 3/4 throttle right away it spit's and backfire's out the TB. on the road it does this taking off from a dead stop but if i feather the throttle a bit and then floor it, it take's off like nothing's wrong and once at speed we start it all over again.

i checked the TPS and it had 4.6v going to "A" and 4.3 at "B",,,i set it to 3.77 at "B" and no change. plug's wire's rotor and cap are all fairly new.

going to check the O2 sensor but i htought this only leaned and richened the mixture, wouldn't cause a backfire through the TB would it ? my thinking is that it's timing that would make that happen but the dist. is tight and hasn't moved. no really bad lifter noise so i doubt it's the cam ( least i hope ). vacuum lines are new, all connected blah blah. i'm stumped and seem to be losing hair now......
 
Just to rule out a valve problem, I would do a compression check.
 
I was reading the FSM on the 2000 the other night (couldnt sleep), and was reading about the sensors the XJ's use. They make it quite clear that even though there are several sensors on the XJ that can affect fuel injection and timing parameters, the MAP sensor was the primary one that had the biggest effect. Not an 89 I know, but I thought I'd just mention it to you...the old RENIX systems might not have been that sensitive to MAP as the newer 2000 FSM leads you to believe. Since I dont have any experience on the older XJ's , I'll let those guys steer you to the problem. Regardless, you've tested the TPS which is always a good place to start.

It seems to me in reading the forums here and other places when someone has a backfire issue that clears out by flooring it like you describe, they have had good luck by replacing the 02 sensor and checking for a plugged catalytic converter. Stalling issues tend to be MAP and or CPS related. No starts seem to almost always be CPS related. Once again, this may only be on non RENIX systems. Its just a general observation I've noticed anyway and certainly isn't intended to be carved in stone.

I've posted on here before about testing an o2 sensor that checked good but was in fact bad. It behaved much like you describe. If you've never changed out one in a long time, you are overdue anyway.

Dave
 
Pussy trouble

Burnt or sticking intake valves are a possibility (but rather unlikely) so a compression check on all cylinders will either confirm it or rule it out.
Bash the underside of the cat (no, not the furry one with four legs and a tail ;) ) with your fist while it's cold. If it rattles like a can of marbles, it's time to change it.
The engine runs well at highway cruise and idles fine so I doubt that the MAP (or indeed any other sensor) is the culprit. A bad O2 sensor would make the engine run worse under these closed-loop conditions and that's not happening, so it's probably still good.
My money's on the cat.
 
My 1988 w/AW-4 does exactly what you are describing. I had a severly cracked exhaust manifold. I just replaced it this weekend, MAJOR! improvement. I also am noticing more power also, always a good thing.
 
converter is almost brand new,,maybe a month old so you owe me some cash :D

i started it this morning and revved it right away and it's all still there. if it wasn't i'd be more inclined to think it was the o2 sensor but with it missing at first start, cold then it rule's that out i believe ?

if this thing had a carb. on it i'd be in heaven, at least then i would know it was timing/distributor or cam/lifter's/valve's...stupid computer crap :repair:
 
jeannies_driver said:
converter is almost brand new,,maybe a month old so you owe me some cash :D

i started it this morning and revved it right away and it's all still there. if it wasn't I'd be more inclined to think it was the o2 sensor but with it missing at first start, cold then it rule's that out i believe ?

if this thing had a carb. on it I'd be in heaven, at least then i would know it was timing/distributor or cam/lifter's/valve's...stupid computer crap :repair:


I agree with you...if its doing it in open loop mode, then its not the O2 (highly unlikely anyway). Lets do the basics first...how about a good plug and wire and cap/rotor change and call me in the morning :D
 
well all the tune up stuff is fairly new but maybe i'll do that tomorrow anyhow.

tonight i went one by one pulling plug wire's and got dramatic changes on all of them so seem's like it's all firing properly.
one thing i did do however just out of curiosity was unplug the TPS unit,,it ran the same way. could this actually be bad even though it's giving me the proper number's ? i'd hate to drop $110 on this thing and be in the same place now but 110 bux poorer as i can't return it.

also as i was messing with it tonight,,if i play with the throttle while under load it goes away " sometimes " and run's fine as long as i stay on the gas. as soon as i let off i'm right back to where i started.
 
jeannies_driver said:
converter is almost brand new,,maybe a month old so you owe me some cash :D

I've already given you my $0.02 so no more dinero ;) , but why didn't you say you had a new cat in the first place?
If the symptoms only occur during the transition from closed to open throttle and vice versa, while the engine runs fine at constant throttle, the TPS could indeed be faulty even though the static voltages are normal.
 
you still owe me money,,changed the TPS this evening and no change ( anyone need a slightly used TPS,,only $80 ) :D
I've already given you my $0.02 so no more dinero , but why didn't you say you had a new cat in the first place?
it has new tire's and washer fluid also.:D j/k didn't think to put it down then, was at my wit's end,,kinda like now,,still.

took out the IAC unit and cleaned it up,, which the way it looked i thought that might be the problem but no luck. at least it's clean now.

just for shit's-n-giggle's i pulled off the MAP sensor and tested it, supposed to have between 4.5v and 5v cold,,this was at 4.95v. so i started it,,let it run a few minute's, put it in gear and held on the accel. a bit just to make sure. engine was warm enough after that ( i think anyhow ) and tested the power feed again. it's still at 4.95v so,,, she's on her way to pick up a new one at the auto part's store ( $39.95 may be for sale shortly also ) so we'll see if that's the problem in about an hour or so :confused1

ever feel like your hasta
 
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Glad to hear you listened and did a compression check. Nothng like throwing money at a problem :laugh3:
 
what would make you think it's a compression issue ? it idle's fine,,run's at WOT fine,,it's just the in between part and it's not all the time. it does it almost every time but not alway's. i can slowly go through the RPM range and not have that much of a problem but if i'm not easy on it and jam it half throttle then it fall's on it's face till i floor it.

oh and no,,the MAP sensor didn't fix it either.
 
jeannies_driver said:
and no,,the MAP sensor didn't fix it either.

I told you the MAP sensor wasn't the problem so I now I don't owe you any dinero anymore. ;)
As Tom said, have you done that compression check yet? My advice was also oriented to help you troubleshoot the problem without throwing money at it. Ah well, you've learnt an expensive lesson.
 
Again,

I had the exact same problem you are having. The 2 issues I had were exhaust related. I had a cracked exhaust manifold, and the vacuum transducer on my EGR valve was full of crud, causing it to stroke too much, and the engine would sometimes quit. I cleaned that out and swapped the exhaust manifold. It now runs smoothly and has significantly more power and no more backfire. Good luck.
 
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