I have (had) a very solid running 97 LT1, it only has 33,000 miles on it. Last summer, the fuel pump died and was replaced. Ever since then, there is a very obvious misfire at idle. On an OBDII scanner, I can see the misfires, and they are random, across all 8 cylinders

So trying to track it down, obviously, looked at the fuel system. The fuel pressure is good, and I had the injectors cleaned. My mechanic is ready to give up trying to track down the problem, because he says we could spend forever trying things. I feel like it's not a fuel problem, that it's actually an ignition problem, because the misfires are so "sharp", like little pings while the engine sits there smoothly idling otherwise. When the fuel pump was dying, it did go through some aggressive engine bucking down the road for a minute or two, so not sure if something could have hurt the optispark or not.

I just feel like there's got to be some way to diagnose it further, at least figure out if the misfires are fuel or ignition related. Any advice on where to start? Thanks so much for your thoughts.

 

So nothing else was touched other than the fuel pump? Car behaves 100% perfect in all conditions other than idle?

I don't think being intermittently fuel starved could harm any ignition components.

I will say that the one time I had my injectors cleaned by a local injector shop a while ago, they gave them back and I noticed a sandy substance in the inlet and outlet areas of the injectors. Not sure what or how that could have happened but they sure looked worse than when I dropped them off. Maybe you have gunk in the injectors that screws with the spray pattern enough that at idle the poor burn quality is more noticeable.

 

Correct, this started at the same time that the fuel pump died and was replaced, and that's all the work that was done at that time. It ran smooth, then this misfire.

I can't say for sure about other RPMs. No misfires show up on the scanner, but I know it's hard to say for sure. The car is a beast and runs great, so if there are a couple misfires at higher end, I don't notice them.

As for the fuel injectors, I only had them cleaned after the misfire had been happening for a month or so. I had them cleaned by a reputable shop that provided me with before and after flow data. As I suspected, they tested good before, and didn't have much room for improvement. So at least I know they weren't skewing the numbers.

Like I said, the misfire is 'clean', like, this winter I had to move the car around in the barn I store it in. It was very cold, and you could hear the misfires clear as day through the cold exhaust pipes, maybe 1 or 2 per second. Just sucks to be sitting at a stop light in a non-crazy-cammed car and have it shaking around.

 

Well, if they're happening on all cylinders you could probably rule out the plugs and wires. That just leaves coil, ICM, and opti. You say it's being stored in a barn. No critters have been chewing on anything? Any way to upload a vid of the car idling? What exactly is the fuel pressure at idle?

 

It was stored this winter, but the problem started before that. I was driving it all last summer, it was running smooth, then the fuel pump died and was replaced, and it immediately started idling with slight misfires. Also, the replacement fuel pump died! So I'm now on a second new fuel pump, and it acts the exact same way, so not a problem with the pump.

I checked the fuel pressure last year, and it tested good at all conditions, but it's been a while, so I don't remember the numbers.

I will try to get a video of it this weekend. Thanks.

 

This is an old post, but I'm still having the frustrating issue. Consistent misfire at idle. The car is very low mileage, the issue began at 25k. I have replaced the coil and ICM, as well as cleaning/testing the fuel injectors, and testing fuel pressure.

I made a video this weekend. I live in the north east, so the car is parked for the winter in my cold garage. It was relatively warm the other day, almost 40, so I fired it up and let it warm up a few minutes before shooting. The pinging noise you can hear is not common on warm days, I think that's just the cold exhaust system, in the summer, those pings sound more like a puff. But you can get the idea of how constant the misfires are here:

https://drive.google.com/open?id=1mS...u_x1AF5FhA-Rvc

My mechanic thinks it might be running rich, and wonders if that's a possible cause, and I'd like to get it tuned, but the tuner guy doesn't want to tune it until the issue is resolved, so I'm kinda stuck in a loop. Obviously, if money was no concern, I'd replace the Opti completely, but if there's no reason to, it would be a huge waste.

Any idea how to go further with identifying the root cause of a misfire like this? Thanks so much!

 

• What testing has your mechanic tried?
• Has he looked at the primary/secondary ignition signals with an ignition scope?
• Is the SES light set?
• Has the car been scanned with a GM tech 2(or up) scanner or a scanner with GM enhanced parameters?
• Does the car feel like it's misfiring under acceleration(if so, when?)

The video does not sound like a misfire to me. It does sound like something is rattling around, but I'm not sure what.
Take a video at the front of the car with the hood up idling. You may also need to data log the car and upload it for us to look at.

 

It's not a dead cylinder at least. If it were you would get a flashing SES light. Next test I'd start it up when it was dark and look under the hood to see if there is any spark jumping off the wires. If there isn't any spark then I'd take the opti off, take it apart and look at the cap and rotor to see what condition they are in. Possible the contacts are dirty. Any backfiring?

 

I know it's low mileage but remember that those ignition components are now 22 years old. Why not just simply tune the thing up? Cap and rotor, plugs and wires. You're not going to spend a ton of money on that and you have the security of knowing things are fresh.

 

Really sounds like a bad wire.

 

I agree with this guy. Sounds like crap in the exhaust. Deteriorated CATs? Maybe?

Also, video heard from front is best.

 

Cats ? Headers?
Extra fuel (even a little) can collect in the exhaust and ignite. I had a similar issue when I changed injectors. I scaled the injectors incorrectly by a little bit.
I tweaked (leaned) it out some and that helped.

Another thing - unrelated but FYI - if you have headers check the temps with a temp gun at each primary shortly after startup. All should be close to each other. That was a good way to find a valve that was not adjusted right.

 

I was thinking similarly or even a plug that’s on its way out. Typically a plug that’s kinda on the verge of fouling or is trying to come back from being too wet will pop through the exhaust. I get this occasionally on my car if it gets too much idle time. For example when I’m not driving it much in the winter I have a tendency to start it and let it run in the garage with the doors up just to let it run. If I do this too much it’ll want to start to foul plugs. Typically I’ll get into the throttle a bit and try to clear it out and it’ll pop through the exhaust until the plug clears itself or just finally dies. Usually it’s one or the other.
That’s why I recommended just tuning it up man. It’s not going to hurt anything and with all those old components it’s really due for it anyway. A quality cap and rotor button preferably with brass terminals and some good plugs and wires. I’d probably go for a set of Delco or MSD wires (I’ve had the same MSD wires on my car for over a decade) and either Delco or NGK plugs are usually my go to.

 

Thanks for all the replies. I know I didn't share a whole lot of information, figured that would be too overwhelming. The car was completely stock, with only 20k on it a couple years ago when I made minor modifications. Here's the setup:

LE1 Heads
Crane 227 Cam
1.6 Rockers
SLP Shorty Headers

Also should mention the y-pipe and dual cats were also replaced at the same time. Immediately after getting this work completed, there were no issues with the misfire I am experiencing now. A couple thousand miles later though, the fuel pump died. The night it was dying, I foolishly drove it too long starved for fuel, and the engine did some really hard bucking and shaking around. Once the pump was replaced and I was driving the car again, I believe is when the misfires began. Obviously, at that time is when I had the injectors cleaned and tested and the fuel pressure checked. Also replaced the pump, because the first one had a manufacturing flaw, but the second one runs exactly the same.

The symptom is 100% consistant. At low RPMs, there a constant misfire. When you sit at a stop light for example, you can feel it sputtering a little bit and the engine shaking around. As soon as I give it gas, it seems fine.

I appreciate what was said in response to my video, how it doesn't sound like a misfire, but like I said, if it were a warm summer day, it doesn't sound like that, that's just the pinging of the cold exhaust system in the middle of winter when the car has been parked for several months. On a normal warm day, it's just a sputtering/puffing misfire sound.

Since the issue arose, the plugs and wires have been replaced, and any possible shorting out of the wires has been addressed. That was definitely an issue to begin with, but I'm confident it's good now. It does not seem like it's any one cylinder that's misfiring from what we can tell, which is why I tried coil and ignition module.

There are no engine codes at all, no exhaust leaks. It's generally a smooth mild running car compared to most, I didn't want a wild cam set up.

ACE1252 - Thanks for the feedback. Most of what you mentioned is outside of my knowledge, and I'd love to find someone around here that would do all of that diagnosis, but I have no idea how to find a shop that I can trust to have that level of understanding. If anyone knows of someone near Rochester/Syracuse NY that has good experience diagnosing these sorts of issues on these engines, I'd love to reach out to them.

AgFormula02 - Thanks for the tip on exhaust temps.

 

So when the cam was installed did anyone inspect the cap and rotor on the distributor? Clearly it came off to do the job, I'm just curious if anyone opened it up and inspected the terminals for corrosion, etc. The fact that it's an OBDII car and it's not setting any codes blows me away. GM cars are notoriously quick at picking up misfires and flashing the light, even at idle. If someone has a good scanner you should be able to access misfire data through the scanner. It'll be able to show you current misfires as well as misfire counts on repeat offending cylinders if you will lol. Have you tried putting a dollar bill close to the exhaust tip while it's running? It's an old-school trick at detecting misfire. Typically when you have a misfire it effects scavenging and when it misses it'll suck the bill up into the pipe.
Also, I'd check to make sure you don't have any vacuum leaks. Vacuum leaks would and will make an engine that typically runs well under throttle idle very poorly. I find this a common problem on newer engines that use a silicone ring type gasket seal. Over time the little gasket dries up and shrinks allowing the engine to draw unmetered air causing a rough idle. Since you've done HCI obviously the gaskets were replaced but it doesn't mean it was a good job or that something couldn't have caused the gasket to fail like over torquing for example. I'd go over all the vacuum lines on it and be sure they're in good condition, no rotted hoses or busted plastic tubing, etc. Whatever you have it's going to be something simple and overlooked more than likely. Worst case scenario would be that you ran it lean when the pump gave up and you've burned a valve which will cause a rough idle but also will typically run fairly normal on the road under throttle. Seen this many times as well as carboned up valves which you shouldn't have since the heads have been worked and were cleaned. I'm just throwing out possibilities.

 

As far as finding someone to diagnose it you can always fall back on your local GM dealer. Yes, I know, no one likes to do that because the shop rates are crazy high but what you'll likely find is there will be an older guy or two there that was working on these cars in their hayday. Those older techs aren't out to screw ya and they're a wealth of information especially on older platforms. The GM dealership I worked at had one guy in particular during my time there and he was my go to guy if I were scratching my head on something or just wanted to chat. Sometimes they're willing to just allow you to come in and talk. Also, getting a good diagnosis from a GM tech doesn’t obligate you to actually have it repaired there. In my opinion, the guy that takes his time with your car and figured out what’s happening to it should get the chance to make some money on the repair and I personally think it’s a bit crappy to let him do the brain work and then take the car but hey, people do it all the time, they do it to Me in the small shop I work at. It feels like a kick in the teeth but at the end of the day you have no obligation to it. You can pay the diagnostic time and be in your way.

 

Last place to take an aging f-body is to a dealership. Owning these cars you need to figure **** out on your own or pay to have someone not knowing what they’re doing throw parts at it while charging $100+ an hour.

 

Maybe I'm your area dude. We actually have some GM techs that know what they're doing here!

 

An area that includes California, three dealerships in TX and Virginia. Stories galore of GM “tech” mouthbreathing wizardry. One of which, the SS was in for warranty work at a Plano dealership and a ******* tried to steal my Hurst shifter, put in a stock one and tried to tell me it was the original, and somehow the Hurst magically reappeared and couldn’t be explained why it was off the car. Another where I had to sue a dealership in Irving, TX over a rotor screw opti issue when they tried to tell me it was a blown head gasket and wanted a CC number so they could do “exploratory surgery” and tried to void my entire warranty.
Point being you’re in a minority if you can trust a GM mechanic. Stores like this are everywhere. Mechanics are in the business of making money and they’ll do and say whatever they can to get the most out of a customer as possible. Especially when it comes to old cars.
Hell, just the other day my friend had his ‘02 Avalanche at a Chevy dealership because of a clunking noise and they tried to tell him not only did he have a bad axle, which is where the noise was originating, but his catalytic converters were also bad which added another thousand dollars to the bill.

 

I found an excellent tech at Robert Woodall Chevy in Danville, VA. I don't know his last name, but his first was Joey. Very talented tech. I enjoyed working with him. After my cost care plus extended warranty ran out, I started working on it myself. The dealership in general took care of me. We bought two or three cars from them. Bought my Z28 from them. I can't remember how I initially contacted them on a Z28, but they called me out of the blue about a year later and told me they believed that they just got the car I was looking for.....Z28, hard top, 6M, one owner, 35K, just traded in....LMAO...and it had the factory manuals in the trunk with a big nail bookmarking the opti section.
Well, needless to say I took note, but ignored it....and have had no opti troubles. I have had misfires on it under heavy load and that turned out to be either plug wires or ignition coil. The coil was 20 year old at that point so I changed it...but I think the wires were the problem. I had some 7.5mm aftermarket wires at the time and they had the most hateful boots I've ever put on a plug. The misfire started after I changed the plugs, so I had a good idea that I had muscled one of the wires too much. I went back to AC Delco wires.....still have them today.

A misfire that only happens when the car is moving can be tough to track down. So constant misfires at idle should be easy to isolate which cylinder(s) are having the trouble. That will help narrow the focus as to what to do next. The info on the car helped. As you have a cam, I'm betting the misfire codes have been disabled. That would explain why you have no SES light. My CC503 will throw phantom misfire codes if not turned off.....so that is a standard thing to do for a cammed car.

Have a watch at these videos. One of the best diagnostics techs I've ever seen. Show this to your mechanic as well if you are not technical enough to understand what Scannerdanner is talking about. Not everybody is.....some of this stuff gets deep into operating o-scopes. Without some exposure to them and electronics....it can be overwhelming. However, this should help you isolate, at the very least, to which cylinder(s) it's happening on.

Your issue might not be ignition....could be fuel injector or something else given when it started, but this should help with the all important direction.