Engine ruff at 1000 rpms

[K1WY]

Any ideas on a ruffness at around 1000 rpms on a 93 LT1?

Car has 30K on it. New plug wires, coil wire. I have a new optispark unit but don't want to do the exchange if not needed.

 

[Ken]

Any OptiSpark fans got some feedback for this Cat?

 

[BlackNBlue95]

I just want to make a general statement.. It's hard to say what is causing your car to run rough.. You really need to do some testing and inspection to get a difinitive answer.. otherwise you are just shooting in the dark at replacing components that might not need be.. If I was looking into this problem I would probably use me shop tools such as a Ignition scope and check the iginition pattern for starters along with a few other tests..
just my .02
-Rick

 

[Lani's 93]

rough @ 1000

I had (have) the same problem with mine. Historically, I don't remember prior to having the water pump/opti replaced if it was there before, although I believe I would have noticed it. Anyway about 2 weeks ago my water pump developed a leak and I took it into Chevy to have it replaced and replace the opti while they were there. A year ago I had new stock plugs and wires installed. When I got the car back it had a very rough condition at exactly 1,000 rpms. But I also noticed it was very difficult to get the accelerator pedal right at 1,000. Anyway took it back they looked it all over couldn't find anything, then figured the opti was bad and took it all apart again and replaced the opti. Got it back and no different, they claimed they worked all day with two corvette guy's and could find nothing that would be causing it, even Tech II said it was fine.

I brought the car home was looking over the engine to what they did do, and noticed the accelerator cable was not on the cam lobe that feeds it to the throttle body. It had been knocked off while they were repairing it and it was just hanging loose there. So I fed it back on, got in, started it up and it was very easy to bring it to 1,000 and 90% of the roughness was gone. I do notice just a hint of roughness now especially when your on city roads where the speed is like 35 or so and your trying to maintain it in the normal drive position. That just happens to be right at 1,000. I suspect maybe a sensitive wire, coil, or fouled plug. The idle and above 1200 rpms is perfect. Hey it seems even peppier with the new opti. Or again that hint of a miss which I now have could have been there before or maybe not I'm not sure. I suspect now that the slop in the cable was causing it to be so difficult to apply a small amount of force to bring the engine to 1,000 and was causing fluctuations making it run rough.??

Anyway stranger things have happened, so you might just want to look to make sure that your cable is ok. Sounds bizarre but its true. Hope this helps.

Lani


I want to be fair to anyone reading this post and add a follow-up. Well that was not the problem, although it did resolve the issue with the accelerator being difficult to hit a specific low rpm range and the accelerator is much smoother. But it did not get rid of the miss, the hint of one I mentioned above is becoming more aggrevated.

I realize to some degree there is sophistication in a 93 C4 setup, but one would think that there should be enough sensor information to isolate obvious misses in engine performance to possible suspect components. I'm not familiar with what a Tech II is, and as well I believe the 93 did not have the newer enhanced diagnostics software, but one would think that chevrolet technicians who supposedly are corvette experts should have enough tools and knowledge to isolate and fix such problems. Maybe I'm assuming it is too much. But for $1260. I don't think so.

All I know is the engine was perfectly fine before the water pump started seeping and it was immediately taken in to the dealer. All they did was remove and replace the opti-spark and water pump. Now it's not well at all. Running it at that 1000 rpm range to maintain 35 mph speeds (normal local city streets) it is really running rough, and at nuetral/park idle bringing it up to just a hair above 1000 its an obvious miss. But just idle and other rpm operations seem perfectly normal.

Sorry about this, I realize I'm venting and its very frustrating. I'm sure everyone who has experienced it understands that it 'sucks' when one's vette is not running right and the fix is not obvious. The situation lends itself to possibly replacing all kiinds of components that are not necessary. I sure wish there was some experienced corvette technicians or bowling green engine engineers here who might read this and offer some assistance. I've read several similar issues others are having but no one seems to ever resolve it. Can there be that many poorly running C4s on the road or parked in garages? Gosh I hope not.

So my apologies if these types of posts are annoying, but I wanted to follow-up so as to not mislead anyone that something so silly as a cable fixed it.

Regards, Lani

 

[poorboys831]

hi lani-just read your post all the way thru and i have a similiar situation.I replaced my opti and water pump and have acquired a very slight miss also.it's been acting up from the time of that change till now.my miss is actually at 800 rpm's,just at idle.i'm kinda lazy and been working strange hours and haven't really had a chance to check out everything real well-i kinda think i have a loose spark plug wire. i'm definately getting under the car this week-end and trace those wires.i'll let you know the outcome-i did the work myself and i know how easy it is to think you have every thing together and just didn't push hard enough on one thing all the way-did they re-check your wires? keep wavin'dennis

 

[Lani's 93]

yea supposedly, they had it a full day with it after they did the first replace and claim they couldn't find anything wrong, so it was back the next to have another opti installed, still the same. The kept it that whole day and said no codes no nothing they checked everything (except for the cable off the lobe).

Mines right between 1050 and 1100 rpms. There is a consistent miss too while its idling, but that's livable, its the rough running like a cylinder or two not firing that's drivng me wacko...and that rpm range is what's needed to maintain about 35-38 mph.

I figured I was going to make some time Sat. to see if I can get at and remove the plugs and take a look at them (they were the new ac platiums installed a year ago which are suppose to be for 100K like the original and new plug wires). I still don't see how one gets access to the right side of the opti wires, the darn thing is behind the crank pully.

The only think left I can think of is the coil but they said it was ok. I removed the battery cable today and let it sit for a while to clear the codes thinking it might be something weird like that but noooo. I saw someone comment on a similar post that suggested cleaning the IAC, so I read up on that in the maint manual. The problem with trying to troubleshoot using the manuals is not having the special tools and interfaces. I'm guessing that a Tech 1 or a Tech 2 is their system analyzer machine. It bugs me because I know its not good for it to run that way.

Good luck to you on yours! Lani

 

[poorboys831]

hi again-please let me know how you make out and i'll drop you a line if i find anything that might apply-the problem is similiar but acting up at different rpm's -hope saturday is a lucky day for us both-dennis

 

[MoeJr]

I have never worked on an LT1 before so I have no idea if this may help but is there a crank sensor down on the crank pulley that coulda got contaminated with anti freeze from the water pump leak? I know the crank sensor on my 87 GN was right there. Just a thought.

 

[Lani's 93]

hmmm, I just looked it up and that sensor is mounted on the block just behind the engine mount, and it only shows one on the J engine not the P engine.

if I figure it out, by george I'm patenting the idea LOL

 

[WhalePirot]

A close look at each of the plugs is a great start.

I'd also check vacuum hoses and that IN FACT they changed all wires. I've been billed for work not done.

Too bad you have to stay on the dealer to have the problem fixed,which you paid for. Don't they drive the cars after a 'fix' to see if it is fixed? It's been obvious from my experience, that they don't. That's part of why I don't take my car(s) to a dealer for repair.

Good luck with the miss. Are you sure it's only at 1000?

 

[Lani's 93]

Yes I have to agree about the quality of Chevrolet service, it stinks. Yes they drove it, and myself and the service manager drove it together and they were aware of it. But they claim they have no idea why its doing it. I still haven't decided whether or not to dispute the charge on the CC. There were a couple of factors that made my decision to allow them to do the work; I wasn't feeling well, I had read horror stories on the forums about getting the opti camshaft coupling off, its my only vehicle, and they offered a lifetime warranty when they do the work. Well so much for lifetime warranties. I should have just drove it with the leaking water pump, bought the parts and did it myself. Live and learn.

The plug wires were changed a year ago. Yes they were new as the old ones specifically the one wire that is near the oil dipstick, it had been contaiminated with oil and had begun shorting there. I can't say for sure if they did replace the plugs which they were paid to do, I guess I'll find that out when I remove them. It wouldn't surprise me if they did charge me and never did the work. I know of no honest car dealer, even those who sell Chevrolet Corvettes. But then again I should keep my opinions of them to myself or risk offending those here that are dealers and or work as technicians for dealers and then have the forum administrator ban me for stating a true and honest assessment as an opinion.

Well as I said there is a minor miss at idle, but nothing like what is happening at between 1050-1100 rpm's. The roughness can best be described as a mis-firing cylinder. And the term rough means its obvious how the engine shaking and vibrating from the miss.

I'm sure eventually I'll figure it out. It has to be something they did, since it was perfect before they touched it. And like any car dealer they have no moral responsibilities regardless of any written warranty.