Stumble/backfire on ACCELERATION - help this 84 SE owner
06-20-05, 07:34 PM
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Stumble/backfire on ACCELERATION - help this 84 SE owner
This just went up on the rec.autos.rotary newsgroup. I'd never heard of this kind of problem, and did a search here hoping to find some helpful info.
Nada.
He then posted this up a little later...
Who can shed some light on this guy's problems? 
Nada.
I've run through the things I can think of and am starting to run out of options - hoping someone here can help...?
It's an '84 SE, stock w/ 140K mi. Sometimes on just cracking the throttle open slightly in say a low speed cruise state, the engine stumbles badly and then lets out one or several VERY loud backfires.
Note this is NOT to be confused w/ the milder backfire that many of us encountered over the years w/ various 12As and 13Bs upon decel from a high RPM state - it's MUCH louder: Imagine AC turned on, stereo up, anda large caliber pistol going off right outside the window. And it doesn't happen upon decel.
For starters I am having a hard time determining for sure if it's ignition or perhaps a fuel problem. It has fresh cap, rotor, plugs, wires and coils. The engine is tuned up and dead on to spec including the TPS. It idles fine, starts easily, drives great and most of the time runs like it's always run over the years. But once in a while almost randomly there's this sudden stumble followed almost always a loud backfire (I can avoid the backfire if I'm quick to lift off the gas until the stumble passes...). The stumble only lasts a couple few seconds and a passenger could easily confuse it as a bump in the road.
A) One area I haven't fully investigated are the ignition modules or 'igniters' on the distributor. Last night I tried to swap out the trailing/leading with each other to see if that might make a difference. If anything I think it's *worse* with them swapped. Before I could drive 30 minutes or so w/o maybe one incident. With the module positions swapped I'm having it happen quite a bit more frequently (to be fair I also put on a set of new plug wires so I should eliminate that recent change).
B) Ignition advance...? The rotor advances easliy by hand when I'm playing around under the hood. But I haven't looked closely at it...
C) Anything in the area of fuel delivery? I replaced the fuel filter maybe a year or so ago - My experience here is that fuel related problems cause hesitation at higher RPMs but not at low RPMs and usually don't result in misfires leading to a backfire (backfiring would suggest plenty of unburned fuel vapor is entering the exhaust anyway).
D) EGI Computer related...?
E) Other...Anyway I'm leading towards replacing one or maybe both ignitors but before I blow that kind of money on a 'possibility' am looking for any and all suggestions or input.
It's an '84 SE, stock w/ 140K mi. Sometimes on just cracking the throttle open slightly in say a low speed cruise state, the engine stumbles badly and then lets out one or several VERY loud backfires.
Note this is NOT to be confused w/ the milder backfire that many of us encountered over the years w/ various 12As and 13Bs upon decel from a high RPM state - it's MUCH louder: Imagine AC turned on, stereo up, anda large caliber pistol going off right outside the window. And it doesn't happen upon decel.
For starters I am having a hard time determining for sure if it's ignition or perhaps a fuel problem. It has fresh cap, rotor, plugs, wires and coils. The engine is tuned up and dead on to spec including the TPS. It idles fine, starts easily, drives great and most of the time runs like it's always run over the years. But once in a while almost randomly there's this sudden stumble followed almost always a loud backfire (I can avoid the backfire if I'm quick to lift off the gas until the stumble passes...). The stumble only lasts a couple few seconds and a passenger could easily confuse it as a bump in the road.
A) One area I haven't fully investigated are the ignition modules or 'igniters' on the distributor. Last night I tried to swap out the trailing/leading with each other to see if that might make a difference. If anything I think it's *worse* with them swapped. Before I could drive 30 minutes or so w/o maybe one incident. With the module positions swapped I'm having it happen quite a bit more frequently (to be fair I also put on a set of new plug wires so I should eliminate that recent change).
B) Ignition advance...? The rotor advances easliy by hand when I'm playing around under the hood. But I haven't looked closely at it...
C) Anything in the area of fuel delivery? I replaced the fuel filter maybe a year or so ago - My experience here is that fuel related problems cause hesitation at higher RPMs but not at low RPMs and usually don't result in misfires leading to a backfire (backfiring would suggest plenty of unburned fuel vapor is entering the exhaust anyway).
D) EGI Computer related...?
E) Other...Anyway I'm leading towards replacing one or maybe both ignitors but before I blow that kind of money on a 'possibility' am looking for any and all suggestions or input.
When igniters are suspect do they simply 'fail' or can they become'intermittent' on Rx7s?
My experience w/ GM's HEIs is that they simply work or don't work with no warning whatsoever, never intermittent. WHen they go, they go (and you are stranded) and that's it. But I've never had a problem w/ Mazdas over the years (see my other post). Do they behave in a similar fashion or can they behave in a more 'intermittent' way?
My experience w/ GM's HEIs is that they simply work or don't work with no warning whatsoever, never intermittent. WHen they go, they go (and you are stranded) and that's it. But I've never had a problem w/ Mazdas over the years (see my other post). Do they behave in a similar fashion or can they behave in a more 'intermittent' way?
Last edited by Amur_; 06-20-05 at 07:39 PM.
06-23-05, 07:53 AM
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Can only be
two different areas in all likelyhood. Fuel, or spark. Yeah I know, duh!
I would check the timing advance with a timing light on the car, not just by turning the rotor with the cap off. The advance curve may be screwed up - e.g. broken/weak or sticking advance spring, screwed up vacum advance....
Hm, if its an -se, I wonder if a leaky fuel injector would dump a bunch of gas in at partial throttle, the puke it out the exhaust where the hot cat would ignite the vapours - pow!
If it was a boinger, I would check the timing chain (STRETCH). Does the 13b dizzy run off of a chain driven gear? (never had it apart so I don't know) He says the advance moves, but did he confirm the timing is still set correctly?
He doesn't say exactly how he switched igniters, but if he just swapped the plug from one to another, he just reversed the leading and trailing so it WOULD run different. If he removed and swapped ignitors, that's a different story. Then I would be betting on distributor/timing/igniter and forget the possiblity of a fuel problem.
Easiest thing to try since this seems to be a problem that has slowly developed rather than a problem provoked by changing anything on the car, I would try and get a whole used dizzy and put it in without changing anything else and see what it does.
If that doesn't change anything, I'd pull the injectors and get them to a fuel shop for cleaning and testing. I really don't think fuel filter or pump is the cause or he would have stumble/backfire or some other symptom elsewhere in his power curve.
All the above is based on the presumption that he has't changed something ont he car, and this problem is a result of that change.
Just re-read his note and he mentions changing ignition wires etc. Any chance at all he put the dizzy cap on wrong (yes I know its hard to get wrong), or miswired the new ignition wires?
Later Amur_
Rob
I would check the timing advance with a timing light on the car, not just by turning the rotor with the cap off. The advance curve may be screwed up - e.g. broken/weak or sticking advance spring, screwed up vacum advance....
Hm, if its an -se, I wonder if a leaky fuel injector would dump a bunch of gas in at partial throttle, the puke it out the exhaust where the hot cat would ignite the vapours - pow!
If it was a boinger, I would check the timing chain (STRETCH). Does the 13b dizzy run off of a chain driven gear? (never had it apart so I don't know) He says the advance moves, but did he confirm the timing is still set correctly?
He doesn't say exactly how he switched igniters, but if he just swapped the plug from one to another, he just reversed the leading and trailing so it WOULD run different. If he removed and swapped ignitors, that's a different story. Then I would be betting on distributor/timing/igniter and forget the possiblity of a fuel problem.
Easiest thing to try since this seems to be a problem that has slowly developed rather than a problem provoked by changing anything on the car, I would try and get a whole used dizzy and put it in without changing anything else and see what it does.
If that doesn't change anything, I'd pull the injectors and get them to a fuel shop for cleaning and testing. I really don't think fuel filter or pump is the cause or he would have stumble/backfire or some other symptom elsewhere in his power curve.
All the above is based on the presumption that he has't changed something ont he car, and this problem is a result of that change.
Just re-read his note and he mentions changing ignition wires etc. Any chance at all he put the dizzy cap on wrong (yes I know its hard to get wrong), or miswired the new ignition wires?
Later Amur_
Rob
Last edited by Whanrow; 06-23-05 at 07:56 AM. Reason: add sentence
06-23-05, 05:48 PM
#7
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Originally Posted by Whanrow
Later Amur_
Rob
Rob
Sorry about that, but between work, the new rotary club and repairing the Ottawa Meet damage to my car, everything else fell by the wayside... I'll post to the newsgroup that he should check this thread.
Thank you all for the replies!
Oh, and he has since posted this:
Yeah I've looked too and didnt' find much online info that quite matched this problem.
Anyway to follow-up I bench tested one igniter and it seems OK though that didn't surprise me - I can drive the car for 15 minutes w/o any problem. The bench testing involved wiring up the igniter to various hot wires and a ground and 'switching' one 12V source and checking w/ a test light. Within the limitations of an old test light and aligator clips and touching two wires as a 'switch' it did OK. There were a few times that the test light 'lingered' on or did light up quickly or brightly but remember it was a rigged 'switch' sooooo I can't honestly say this was the most ideal test condition.
The magnetic pickup gap is about .6mm on the leading side - within spec of about .5-1mm.
The resistance across the pickup coil leads on the leading side was about 720ohm (650+-50 is spec) - close to spec, I doubt there's a serious problem there.
Back to the fuel / intake side of things...
- Virtually everything wear / tune related short of the igniters has been replaced.
- I can sometimes kinda 'induce' it by feathering the throttle just above idle while cruising at a modest speed. Say 4th gear oh around 45mph and just give it a little gas and STUMBLE, POW!, Vroom...
Anyway to follow-up I bench tested one igniter and it seems OK though that didn't surprise me - I can drive the car for 15 minutes w/o any problem. The bench testing involved wiring up the igniter to various hot wires and a ground and 'switching' one 12V source and checking w/ a test light. Within the limitations of an old test light and aligator clips and touching two wires as a 'switch' it did OK. There were a few times that the test light 'lingered' on or did light up quickly or brightly but remember it was a rigged 'switch' sooooo I can't honestly say this was the most ideal test condition.
The magnetic pickup gap is about .6mm on the leading side - within spec of about .5-1mm.
The resistance across the pickup coil leads on the leading side was about 720ohm (650+-50 is spec) - close to spec, I doubt there's a serious problem there.
Back to the fuel / intake side of things...
- Virtually everything wear / tune related short of the igniters has been replaced.
- I can sometimes kinda 'induce' it by feathering the throttle just above idle while cruising at a modest speed. Say 4th gear oh around 45mph and just give it a little gas and STUMBLE, POW!, Vroom...
Last edited by Amur_; 06-23-05 at 05:51 PM.
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06-23-05, 07:09 PM
#8
84SE-EGI helpy-helperton
If I were him, I'd check the right side wiring harness for any breaks or worn insulation which could be shorting out the fuel injector(s). If there's a worn area that's allowing the fuel injector to be shorted out, if the wiring hits a 12v hot circuit, it will hold the fuel injector OPEN for as long as the short is held. This would lead to excessive fuel dump which would not be efficiently burned during the combustion cycle. Further, I would surmise that the 'stumble' that he's feeling just before this occurs is a short duration impact of the 'running-on-one-rotor' symptom of fuel injector or harness problems.
When my 84SE had the running-on-one-rotor issue, I could grab the harness behind the water pump and jiggle it around and cause the engine to run correctly and/or run on one rotor. This was due to an intermittent short in one of the fuel injector pigtails that would cut out when the harness was moved. Borg Warner replacements at $7 each (x2) and splicing these pigtails into the existing harness fixed my problem for good.
My recommendation,
When my 84SE had the running-on-one-rotor issue, I could grab the harness behind the water pump and jiggle it around and cause the engine to run correctly and/or run on one rotor. This was due to an intermittent short in one of the fuel injector pigtails that would cut out when the harness was moved. Borg Warner replacements at $7 each (x2) and splicing these pigtails into the existing harness fixed my problem for good.
My recommendation,
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