Author Topic: Misfire Problems  (Read 263 times)

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Offline RomanChariot

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Misfire Problems
« on: January 10, 2019, 08:45:24 am »
My 7.4 liter Suburban has always had a low RPM misfire. It used to be just an occasional hiccup. Recently it began to be a much more severe misfire that happened at all RPMs but it was intermittent. One time I would drive it and it would be fine and the next it would be terrible. Finally, over the holiday break it died and I couldn't get it to start back up. I took it to a local shop and they pulled the upper plenum and replaced all of the fuel injectors (refurbished) as well as the fuel pressure regulator, camshaft position sensor and the spark plugs. I picked it up yesterday and it runs well except for when it is idling in gear and then it has a noticeable misfire. I only drove it a short distance yesterday so I am going to try and drive it more tonight to see if the misfire shows up in other scenarios and then I plan to take it back to the shop.

Any of our mechanics care to weigh in on what they think it might be?
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline shp4man

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Re: Misfire Problems
« Reply #1 on: January 10, 2019, 08:50:13 am »
Bad coil possibly. Any codes? Mode 6 PCM data may tell what cylinder it is. What year is it? How many miles?

Offline CaminoRacer

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Re: Misfire Problems
« Reply #2 on: January 10, 2019, 08:56:00 am »
Sounds like a coil.
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Offline giant_mtb

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Re: Misfire Problems
« Reply #3 on: January 10, 2019, 09:06:21 am »
Jesus, your mechanic did all of that before starting with checking for proper spark (ie, plugs/wires/coils)???

Offline RomanChariot

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Re: Misfire Problems
« Reply #4 on: January 10, 2019, 09:11:59 am »
Bad coil possibly. Any codes? Mode 6 PCM data may tell what cylinder it is. What year is it? How many miles?


Sorry, I forgot to mention that it is a '99 Suburban with 170,000 miles. Although it is OBD2, it never once tripped the check engine light with all of the misfires. The shop said that the OBD2 system in that truck gives very limited information.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline RomanChariot

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Re: Misfire Problems
« Reply #5 on: January 10, 2019, 09:16:04 am »
Jesus, your mechanic did all of that before starting with checking for proper spark (ie, plugs/wires/coils)???

From the research that I have done the fuel pressure regulator is a common failure and some of the symptoms that I was seeing before taking it in matched a bad regulator. Bad fuel injectors are also a common cause of misfires. They had to pull the upper plenum to check the regulator and the injectors. It wasn't until they had tested all of those items that they told me what needed to be replaced.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline shp4man

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Re: Misfire Problems
« Reply #6 on: January 10, 2019, 09:23:52 am »
Sounds like an expensive repair you've already done. If you take it back to them, they should at least attempt to figure it out. Now if it's determined that there's base engine issue, that should have been eliminated as the cause before they sold you a shitload of injectors and crap.
At 170K, any competent tech would have upfronted the customer about possible expensive engine repairs/replacement.
Did they do or recommend a compression test?
If it's a coil, that's understandable with a bunch of other shit masking the problem.

Online Soup DeVille

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Re: Misfire Problems
« Reply #7 on: January 10, 2019, 09:40:04 am »
They shouldn't need to pull the plenum just to test the regulator. The should be able to test the pressure before and during running to see if it stays in spec; if they need to change it, they need to change it, but they should be pretty darn certain before they pull that much apart.
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Offline RomanChariot

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Re: Misfire Problems
« Reply #8 on: January 10, 2019, 09:50:33 am »
Sounds like an expensive repair you've already done. If you take it back to them, they should at least attempt to figure it out. Now if it's determined that there's base engine issue, that should have been eliminated as the cause before they sold you a shitload of injectors and crap.
At 170K, any competent tech would have upfronted the customer about possible expensive engine repairs/replacement.
Did they do or recommend a compression test?
If it's a coil, that's understandable with a bunch of other shit masking the problem.

I don't know if they did a compression test or not. The engine has actually run really well. I have had it for 10 years since buying it with 125,000 miles on it. I use it for pulling a travel trailer with a family of 8. There are not very many vehicles that can do that job.

Normally I do all of the work on it myself but it was 25 degrees out when it broke down and I knew that I was likely going to have to pull the plenum off which is a time consuming affair so I decided to take it to a shop that my father-in-law uses. They have actually been reasonable on their rates and didn't mark up the part costs substantially like some shops do.

I suspect that once they got the engine running good in the shop they called it good and did not do a thorough test drive to noticed the current miss. I am planning on taking it back in but I thought I would get some feedback from the brain trust first. I appreciate your input.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline FoMoJo

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Re: Misfire Problems
« Reply #9 on: January 10, 2019, 10:08:28 am »
It would seem that the engine dying problem was apart from the initial misfire problem.  Seeing that the only things that weren't changed were the coils I would think that checking them and replacing suspect ones would resolve your misfire.
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein

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Re: Misfire Problems
« Reply #10 on: January 10, 2019, 10:28:34 am »
Lets's throw a cam sensor in there for the hell of it?
How does it run under wide open throttle now? Does it have all of the beans?
1988 GMC K1500 4.3 ... 2017 Ford Fiesta 1.0 ... 1986 Ford Ranger 2.0 ... 1998 Dodge Dakota 2.5 ... May God Rest Their Soul

Offline RomanChariot

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Re: Misfire Problems
« Reply #11 on: January 10, 2019, 10:51:19 am »
It would seem that the engine dying problem was apart from the initial misfire problem.  Seeing that the only things that weren't changed were the coils I would think that checking them and replacing suspect ones would resolve your misfire.

It only has one coil.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline FoMoJo

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Re: Misfire Problems
« Reply #12 on: January 10, 2019, 10:53:36 am »
It only has one coil.
Well, in that case, disregard what i said.  Maybe plug wires?
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein

Offline CaminoRacer

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Re: Misfire Problems
« Reply #13 on: January 10, 2019, 10:56:06 am »
My Mazda6 only has 1 coil but it started misfiring on cyl 4. Then 3 & 4. So it could still be that, or the connection from the coil to the plug wires.
1969 El Camino - AutoXer

Offline RomanChariot

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Re: Misfire Problems
« Reply #14 on: January 10, 2019, 10:56:33 am »
Lets's throw a cam sensor in there for the hell of it?
How does it run under wide open throttle now? Does it have all of the beans?

After replacing the fuel pressure regulator, injectors  and plugs they said that it felt like the timing was off. There is no timing adjustment on that vehicle. You line the distributor up and drop it in and the computer does the rest. That is when they replaced the cam sensor.

I got home late last night after my wife had picked up the Suburban. I only had time to take it on a short drive but it felt good when I got on the gas. I am going to take it for a more thorough drive tonight and see if the misfire shows under any other conditions.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline RomanChariot

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Re: Misfire Problems
« Reply #15 on: January 10, 2019, 10:57:52 am »
My Mazda6 only has 1 coil but it started misfiring on cyl 4. Then 3 & 4. So it could still be that, or the connection from the coil to the plug wires.

I am suspecting either the coil or the ignition module.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline shp4man

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Re: Misfire Problems
« Reply #16 on: January 10, 2019, 10:58:36 am »
Plug wires...

Offline CaminoRacer

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Re: Misfire Problems
« Reply #17 on: January 10, 2019, 10:59:42 am »
I am suspecting either the coil or the ignition module.

I've been stumped by a module before. Didn't even know it existed, since it's hidden inside my HEI distributor. (It's since been replaced with a separate MSD ignition box)

Might as well start with plug wires, then move on to coil/module.
1969 El Camino - AutoXer

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Re: Misfire Problems
« Reply #18 on: January 10, 2019, 11:00:36 am »
Vacuum leak.
1988 GMC K1500 4.3 ... 2017 Ford Fiesta 1.0 ... 1986 Ford Ranger 2.0 ... 1998 Dodge Dakota 2.5 ... May God Rest Their Soul

Offline CaminoRacer

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Re: Misfire Problems
« Reply #19 on: January 10, 2019, 11:01:18 am »
Bad gas.
1969 El Camino - AutoXer

Offline giant_mtb

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Re: Misfire Problems
« Reply #20 on: January 10, 2019, 11:01:20 am »
My buddy's gf's Subie was exhibiting a misfire the other day.  Changed plugs, no change. Checked compression, all good.  Fresh plug wires...problem solved.

Offline RomanChariot

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Re: Misfire Problems
« Reply #21 on: January 10, 2019, 11:01:34 am »
Plug wires...

They are fairly new but always a possibility.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline CaminoRacer

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Re: Misfire Problems
« Reply #22 on: January 10, 2019, 11:02:12 am »
I should say that when my module went bad, the car wouldn't start at all.
1969 El Camino - AutoXer

Offline RomanChariot

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Re: Misfire Problems
« Reply #23 on: January 10, 2019, 11:04:08 am »
2002 BMW 530i
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1999 Chevy Suburban (The Beast)

Offline AutobahnSHO

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Re: Misfire Problems
« Reply #24 on: January 12, 2019, 07:49:53 am »
muffler bearings
Will
 
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Offline CaminoRacer

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Re: Misfire Problems
« Reply #25 on: January 12, 2019, 12:13:44 pm »
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Offline RomanChariot

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Re: Misfire Problems
« Reply #26 on: January 14, 2019, 09:28:48 am »
muffler bearings

I haven't checked the muffler bearings yet but the shop did a full lube job and topped the fluids so the blinker fluid can be be ruled out.

I put the Suburban through the paces over the weekend and wouldn't you know it, not a sign of misfire the entire time. It starts a lot easier and feels like it got some power back.

I really hate dealing with intermittent problems.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

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Re: Misfire Problems
« Reply #27 on: January 14, 2019, 09:38:38 am »
I haven't checked the muffler bearings yet but the shop did a full lube job and topped the fluids so the blinker fluid can be be ruled out.

I put the Suburban through the paces over the weekend and wouldn't you know it, not a sign of misfire the entire time. It starts a lot easier and feels like it got some power back.

I really hate dealing with intermittent problems.

Maybe la computadora just had to adjust to its new parameters?
1988 GMC K1500 4.3 ... 2017 Ford Fiesta 1.0 ... 1986 Ford Ranger 2.0 ... 1998 Dodge Dakota 2.5 ... May God Rest Their Soul

Offline RomanChariot

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Re: Misfire Problems
« Reply #28 on: January 14, 2019, 10:47:29 am »
Maybe la computadora just had to adjust to its new parameters?

One can hope. However the computer in this thing is closer to a Vic 20 than MacBook Pro.
2002 BMW 530i
2001 Dodge Grand Caravan
1999 Chevy Suburban (The Beast)

Offline FoMoJo

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Re: Misfire Problems
« Reply #29 on: January 14, 2019, 11:27:26 am »
I haven't checked the muffler bearings yet but the shop did a full lube job and topped the fluids so the blinker fluid can be be ruled out.

I put the Suburban through the paces over the weekend and wouldn't you know it, not a sign of misfire the entire time. It starts a lot easier and feels like it got some power back.

I really hate dealing with intermittent problems.
Just a thought, might it be affected by moisture?  Something in the wiring leaking a bit.
"The only reason for time is so that everything doesn't happen at once." ~ Albert Einstein