Rough Running, Random Backfire, [ FIXED TRANSMISSION ISSUE ]

1992 Cadillac DeVille 120000 miles. Has a rough idle when sitting still checked for vacuum leaks 2 weeks ago and found a couple lines where leaking that come from the bottom of the throttle body housing back around the front of the engine over to the check valve. We also replaced the check valve. I will throw the information out there as I remember it so some of what I did before maybe later on in this post.

The car had seemed to run a little bit better after that when I looked at it that day until I took it for a test drive it would run rough and act like it was flooded now until I got 225 30 mile per hour then it would take off and just run great as can be.

I wasn't able to hook up to the GM1 with the connector because with my DA5 adapter and Modis Edge scan tool as it was too long to get plugged in so I will see the snap on guy tonight and he thinks he has a OBD1 cable or some type of cable which I hope works because. I'm a little worried because I know the gm1 adapter has a plug in the side which from the best of my knowledge I would have to supply the 12-volt power to that to help the scanner read the codes from the vehicle. This was the same case with the Toyota adapter I had when I had to pull codes Under the Hood on the passenger side near the firewall I had to have a 12 volt plug plugged in to the side of the Toyota adapter.


I did notice when I drove the vehicle before after I was done driving it and I would run the idle up to 2500 rpm and then let off during the time I was running it up I would smell a fuel odor then when I would let off I would hear something on the bank 2 rear of the engine closest to the driver steering column make a clunk and then that order would go away until I ran the idle up again and yes it would run a little rough all running that idle up then like it was flooding out at seemed from what I recall.


Fast forward to last night and my friend sent me a message saying when he tries to accelerate there's no power and that it floats out or so it seems and even will let out a backfire. So here I am scratching my head until I meet but the snap on guy and hope I can get in and at least pull some codes to give me a direction that I need to go. Given that the car is a 1992 I doubt I'll get much if Little live data at all but it would be nice to have some codes to have a sense of direction of which way to go if something is putting a code but there has been no check engine lights either.


Here's what 2 separate texts combined highlighted that I received last night and today,

"Quit running on him last night. It would sputter, backfire and not accelerate in one text msg.

Would not go very fast, putted and wouldn't accelerate in another text. "

Some of the prior history looking at it follows. I didn't experience those symptoms but it has gotten a little bit colder out by about 10 degrees average and it's been a few weeks

I will say last time I did not have a fuel pressure gauge with me as I couldn't find mine but I do have one for when I go back today or should I say this evening in the cold outside and no garage.

What I'm wondering is there some pointers that some of you can maybe give me on what to check when I go back this time? Also I just remembered when I looked at it before I did check the throttle position sensor with the lab scope as one of the component test because the guy said he was told it was a throttle position sensor and it had a nice good curve as increased like it should when you depress the accelerator.


Any and all help will be greatly appreciated. Thanks in advance! D.N.

P.S. please excuse if there are any spelling or grammar errors as I used voice text as it's kind of hard to type this on my mobile device.

Also, if it's still acting the same should I take a video, upload it to my Dropbox account and then share that link to the site to be able to post a video?

Quick update on my post. Check the fuel pressure regulator it's good. Once the vehicle was fully warmed up I would run the RPM's up to 2500 while in the engine bay and after about 10 seconds if I let off I would hear that clunk as distinctive as it was before coming from the rear of the engine. I did it a couple times and it still continue to do it.


I went and sat in the driver seat of the vehicle and did the same thing and when it did that clunk my foot happened to be sitting out the door on the ground and the car is sitting sloped slightly forward on a driveway. When it made that clunk the car moved forward a couple inches while in park gear. I thought the same. So I did it a couple more times and was able to produce the same result every so many clunks.

Now I'm starting to wonder if something is binding up between the engine and transmission causing this to happen. With the 1992 Cadillacs would they have even had sensors back then that would deal with sensing something of this nature maybe a flywheel or torque converter issue?

It seems as if it may be a transmission issue causing drivability issues? The reason I put the question mark is I am seeing if there's a consensus among other fellow text out there that may read this.

It would be nice to hear if anybody has ever experienced this or heard of this. I am going to recommend the guy either take it to a transmission shop or probably junk it out given the age of 26 years old. It only has a hundred twenty thousand miles but I could see this potentially nickel-and-diming him down the road and him taking the money he was thinking of paying for this car and adding a little bit more to it and getting himself a little bit better vehicle probably 10 to 15 years newer.

I hesitate even to post this...
I hate silver bullets, and I always cringe when the first advice somebody gives starts with "change X part", but I punched your car and symptoms into iATN and found an overwhelming number of cases like yours with the same confirmed fix: block grounds. On the starter and alternator.

I wouldn't have had a ground issue in mind right away, but there's too many cases to ignore it. At least for the running issues you describe, the clunk/lurch thing sounds like it could be converter clutch lock up issue of some kind. Automatic transmissions are kind of magic to me....
I don't know how they do it (at least not yet), but they do it.

Noah wrote: I hesitate even to post this...
I hate silver bullets, and I always cringe when the first advice somebody gives starts with "change X part", but I punched your car and symptoms into iATN and found an overwhelming number of cases like yours with the same confirmed fix: block grounds. On the starter and alternator.

I wouldn't have had a ground issue in mind right away, but there's too many cases to ignore it. At least for the running issues you describe, the clunk/lurch thing sounds like it could be converter clutch lock up issue of some kind. Automatic transmissions are kind of magic to me....
I don't know how they do it (at least not yet), but they do it.

Noah, Yes Sir it's funny you post the ground and transmission issue.

The alternator does have a ground and there's also a ground from the block to the firewall back side of engine that has a ...... get ready for this..... household electrical twist connector on it. It works as a ground and I left it. "If it works no need to fix it".

This was a deal a painter friend of mine was borrowing from his mother who had it as a spare she hadn't driven much lately and she'd got a deal on it. I guess she's been thinking about selling it. Probably a little to late for that especially if anyone takes it for a test drive.

I do believe it's the torque converter locking up after I'd spoke with the transmission shop friend of mine and doing a little extra searching as well. The pump us most likely gone as well as the guy said don't even waste your time with a 92 Cadillac which I agree with.

The engine would be a good donor. To bad he had a friend who owns a shop throw alot of parts on it before he came and saw me. Apparently it runs the same as it did before he took it to the shop he'd went to. I spent some time with it, fixed the vacuum leak the last guy missed and ran the rpm's up as Paul shows to do and that's when the clunk reared it's ugly head so sometimes it pays to slow down. Still got paid for the limited amount of work I did and he didn't waste another couple hundred on parts, just about $6-7 on a small section of vacuum line and a bad PCV valve.

Noah wrote: I hesitate even to post this...
I hate silver bullets, and I always cringe when the first advice somebody gives starts with "change X part", but I punched your car and symptoms into iATN and found an overwhelming number of cases like yours with the same confirmed fix: block grounds. On the starter and alternator.

I wouldn't have had a ground issue in mind right away, but there's too many cases to ignore it. At least for the running issues you describe, the clunk/lurch thing sounds like it could be converter clutch lock up issue of some kind. Automatic transmissions are kind of magic to me....
I don't know how they do it (at least not yet), but they do it.

Noah, After doing some more digging you might actually be onto something and definitely are regarding the ground as it spells it out. Even describes the symptoms of the vehicle. If it's just a solenoid and getting the transmission serviced then by all means it may be worth him having it looked at for an hour at a transmission shop.

I don't mind doing cv axles, seals, external switches, u-joints, axle assembly seals and rear transmission seals but other than removing one from an engine and the stuff above I steer clear of them.


Here's the link about the GM Transmissions. Sounds as if it's a common thing. Hopefully not all GM transmissions have this issue though.


www.thoughtco.com/diagnosing-gm-converte...k-up-problems-262653

Admins you can mark this post closed/fixed.

Transmission issues.