Hi guys, I am a new member but I have visited the site regularly for years. I have enjoyed and learned a lot from reading posts. I hope I have posted this thread in the right place.

Hopefully, someone can shine a light on a problem I am having that has stumped me.. I am working on a 1996 lt4 Corvette. The car has 55,ooo miles. It was bought not running due to a bad opti. I replaced the opti with a MSD pro-billet dist. (per owner request). Got it to run but had three injectors out from sitting. Replaced factory 28lb injectors with 30lb Accell injectors.

Here are the problems: 1. Intermittent stumble on low rpms and occasionally while under hard load. Very difficult to reproduce as it seems to only do it when it wants to but is fairly often. 2. After a reasonably hard pull or WOT pull through a couple of gears, and it is placed into a higher gear to cruise, it shakes violently and feels like a severe miss. It does not do this when driving easy. 3. Revs sort of sluggish (my stock 97 T/A revs way better). No codes are present. I have an OTC Genesys scan tool and it does not show any missfires occuring. I have searched and searched and cannot find my exact problems, but I am leaning toward a faulty opti as I understand the MSD's do not have a great reputation.

The car has a previously installed hotcam and longtube headers. I do not know how it ran before the opti failure as it was purchased not running.

Here is what all has been done recently: New fuel pump (locked up from sitting), MSD Pro-billet dist., MSD coil, ICM, MSD wires, injectors, 02 sensors, and probably more that I am forgetting. I have done all the basic stuff. I have been a mechanic for 17 years, so I don't think I am overlooking something stupid (always possible though). Does anyone have any clues? At this point, my next step is to tear it back down and swap the opti with a used stock one that I have. I really hate to do that if I don't really need to. Any help from someone that has been here is greatly appreciated!

One other note: It was picking up KR on my scanner. I have tunercats and I disabled the KR and still have the same problem. Sorry for the long write up, I just want to provide as much info as possible. Thank you all!

 

Sounds like spark is weak. You will notice it more when under a load. I'd check to make sure coil wire is good and then maybe put the stock coil back on for a test. The MSD coils sometimes fail. A lot easier to change the coil than the opti.

 

I agree, I have already tried it with the stock coil with no change. I put an in-line spark tester into several different wires and it was getting a good, hot spark. I can't remember what I had the tester set at, but was jumping a very large gap. It just seems like something is breaking down under a load when I can't test it.

 

Have you tried swapping the ICM. They have a low and high load. As far as I know this is still one of the very few that can't be tested. If you haven't tried it yet, swap it.

 

So you didn't retune when you added the bigger injectors? Sounds like that may be your problem, does it smell rich?

Before you installed the opti, did you take it apart and loctite all the screws? I did to my MSD opti when i installed it 7 years ago and have never had an issue. Most often this is why new optis fail, although in this case I think your bigger injectors are causing your problem.

 

You guys are awesome! Thank you all for the thoughtful responses.

I replaced the ICM after the problem was noticed. At first I stole the one off of my T/A because it was late on Sunday, I replaced it with a new one on that Monday. Neither one provided any change.

I did re-tune some after the injector install. I started with changing the injector constants (used the formula to figure it out, but can't remember now what it was)-no change. I have leaned out the idle, richened the top end, eliminated KR, added timing, plus some other things all with no change to the original problem. It runs much, much better when it is not messing up-it now pulls hard all the way to 6500 and with the stock program it nosed over at about 5600. All of the changes were done a little at a time. Once I realized that I was not making any headway with tuning, I re-loaded the stock program thinking that I have a hard part failure and wanted to correct it before attempting to tune.

 

Sorry, I didn't answer all of your question. Yes it is rich (at least at idle), but leaning out the VE tables and changing injector size had no effect. I was tuning mostly in SD mode to bypass MAF problems. Shamefully, I do not think I locktited the bolts. I really can't remember as I installed the opti last fall. I didn't realized until after I started researching this problem that was an issue. The car has only seen about 100-150 miles since the opti install. Many, Many trips around the block test driving and tuning. I wouldn't think they could have worked loose that quickly even if I didn't?

 

I'm in the exact same situation with my 93 currently. I'll be installing the third opti in a months time tomorrow. The only real difference is I'm running the delteq coil setup, but I did notice that the stumble has gotten extremely more noticable with a loose converter. Hopefully this opti will cure my problem and give you a direction.

 

Thanks duh, let me know how yours turns out. It sure looks like I am doing this job again. Murphey's law is always there by me to ensure I get the bad one out of the box. I have read a lot about the MSD bolts..has anyone had similar problems with one breaking up spark right out of the box? If in fact it does turn out to be the MSD, I really hate to put another one on it. At $500 I have to use the one year warantee though and get a replacement.

Are you having the same symptoms?

Does anyone else have any suggestions before I tear this thing down again (maybe a magic wand or ferry dust)?

 

It was not previously tuned. I reloaded the stock tune for the time being. I can tell an obvious difference in performance with the changes I made compared to stock, but the problems go unchanged no matter which tune is in it.

 

Not to be a dick, but do you know what you are doing when you tune a LT1 PCM? Unless you are an expert, leave it to the professionals and get a mail order tune, from PCM for less or another REPUTABLE tuner it's going to be very hard to pinpoint the issue unless you have a good baseline tune to go on. My advice would be to do what the owner should have done in the first place before he put a cam in it: GET A TUNE!

 

The hot cam doesn't "need" a tune to run in an lt1. To get the most out of it, it does. The cam is very close to the factory specs. Fuel injection is just doing the same thing a carb does, only electronically. For that minor of a cam change on a carb car, you'd most likely keep the same jets and idle screw settings. It's not that big of a change. I'm with the OP in that its opti related. Only reason I say that is my car has neither a tune or a cam, and it has damn near identical symptoms. I'm putting in the new opti tonight, so we'll see how it goes.

 

Yea it does....if you want it to run good.....he's changed the injectors too....how does the PCM know that? It doesn't until it's tuned. The car was never tuned properly to begin with.

Also, the factory cam is quite a bit smaller than the LT4 hotcam...very close?!

LT1 Stock: 202/207 0.450/0.460 lift 116 LSA


LT4 stock 203/210 0.476/0.479 lift 115

Hotcam: 218/228, .525/.525 Lift 112 LSA

 

The injectors will create havoc. I didn't say they wouldn't. The hot cam on the other side, when created, was advertised as not "requiring" a tune. The car will run fine without one. Again, to get the most out of it, it will need to be tuned. You have to actually read what people are saying. In the world of camshaft tech, the hotcam is damn near considered a stock cam. Most early performance sbc had larger specs. Consider the infamous DZ302.... 254/254 @ .050 .485/.458 try running that little solid roller with our ECM and the computer will go retarded if left untuned. The hot cam is a baby in the world of performance. That's why when GM sold the kit, it did not say a reprogram of the ECM/PCM was required.

 

It wasn't REQUIRED but tuning was highly recommended. I used to have practically the same cam as the hotcam (lunati voodoo 221/229) in a cam only setup...I would disagree it's "practically stock"....HUGE difference in power, sound, torque curve. Maybe something along the lines of a crane 227 is "practically stock"...that's a real baby cam; you could get away with no tune on that. You aren't fooling anybody with a hotcam...they will know it's cammed

 

Well, I certainly didn't want to start any arguments. Am I an experienced lt1 tuner...no-is anyone? I obviously do not have a dyno, so there is a lot of trial and error when I make changes. I have tuned an lt1 with a comp cam much larger than the hot cam and as blown GTA implies, I have a supercharged 355 tuned port in a 91 GTA that I have tuned. It is a never ending process for someone as picky as i, but i have had good results. I have not blown anything up yet, and I improved power and drivability (probably not as much so as a pro I agree. The lt1 with the comp that i was speaking of was in a 94 Z, and it to had 30 lb. Injectors. When we first put it together, it had trouble idleing, and didnt make sufficient power, but did not buck and jerk like this this lt4. Assuming that i have no clue what i am doing tuning (fair enough), i have made some drastic changes to fuel, and timing changes with NO change in the stated problems, even though wot screams now (when not popping back). I wish I was the one that put the cam in, or at least drove the car prior to the opti failure, but not so fortunate. It would be easy to diagnose if I put a cam in it and then the problem started, or was fine with cam and started acting up after opti.

Duh, let me know how yours turns out. As much as I would like to work on the Vette, I had to pull it out to rebuild a stupid SER sentra. I will get back on it mid next week or so when this stupid rice burner is out of my garage.

 

Well, I had another DOA opti last night. Car hasn't started since Tuesday. I'm to the point where I'm going to deal with the driveability issue for a bit, since I'm tired of wrenching this week. Upside, I can officially have the Opti off the car and stripped down in 45 minutes.

 

Thats pretty quick. This corvette is a real pain in the @$$. There is a lot less room between the radiator, plus it has a camber brace in the way....oh well. None of them are fun, but i would rather do one on my 97 T/A. Good luck with yours.

 

30lb injectors? Hot cam? No tune? I'm surprised the thing will still start. Get a tune before it dies.

 

i would say that opti is probably the issue as well. But as a lot of people have said try getting a pcm4less or one of the people who do lt1'a constantly.. give em whats done and they will set it all up correctly. See how it works then head to the mechanical side maybe even while you have the opti off pop the timing cover and make sure they have the cam degree'd correctly it may be something like that, Especially since a lot of people have dreams of big power but don't read or have enough experience to pull of a process like a cam swap, Lashing valves, ect. I am also having a spark/shitty running lt mines in a 95 formula, I think i maybe looking at an opti aswell so keep us posted as to what you do/figure out!