Darkgh0st

I'm having an issue with knocks being picked up @ 2000RPM. Spark is being retarded by 2 degrees.One example is going up a hill at 30MPH and 2000 RPM i got 22,714 knock counts in 1 min. The problem is intermittent. This is causing major low-end hesitation and major frustration. Any Ideas??

EDIT: It's a 1993 LT1

MODS:
LT4 Hotcam
1.6"RR
Trickflow pushrods
Cloyes Timing Chain
Ported & Polished heads
Magnaflow Mufflers
K&N Filter & Open Lid
Dyno Tune


I posted my Datamaster log HERE. Skip to record #4000 where you see all the spark retard

Any help would be greatly appreciated

VenkmanP

Damn. I opened it in Datamaster EE and thought your car must have exploded. I guess you have a 93...

Darkgh0st

Sorry about that. It is a 1993


DOWNLOAD DATAMASTER FOR 93 VETTE HERE

Beth396

I'm having an issue with knocks being picked up @ 2000RPM. Spark is being retarded by 2 degrees.One example is going up a hill at 30MPH and 2000 RPM i got 22,714 knock counts in 1 min.

The problem is intermittent.

What do you mean by intermittent? Do you mean that you only get knock counts at 2000 RPM and low vacuum conditions (high MAP)?

This is causing major low-end hesitation and major frustration. Any Ideas??

Have you had this problem only since you've made the mods? If so...stating the obvious, back off on the spark advance in the area of the spark advance table where you're getting knock (2000 RPM, high MAP). I didn't look at your file. What is the spark advance during this condition?

The more detail you give, the better the chances someone can help you.

Darkgh0st

What do you mean by intermittent? Do you mean that you only get knock counts at 2000 RPM and low vacuum conditions (high MAP)?

What I mean is that I can drive the car 1 day and it will drive completely normal and other days its hesitating like crazy.

Have you had this problem only since you've made the mods?

Yes. There were no problems before. After the mods it ran great for 2 years and just started happening slowly.


If so...stating the obvious, back off on the spark advance in the area of the spark advance table where you're getting knock (2000 RPM, high MAP). I didn't look at your file. What is the spark advance during this condition?

I have no idea. This was originally tuned with a "best guess" tune by my engine builder when he did the top end work. It has since been dyno-tuned (same person) and it the problem didnt get better or worse with the new tune. The tuner said that he didn't see anything abnormal.

I'm very limited in my knowledge on this stuff so I can't really say anything about MAP maps and stuff (which is why I posted my datalog)

Beth396

There were no problems before. After the mods it ran great for 2 years and just started happening slowly.

Than it probably isn't the tuning. When is the last time you pulled the spark plugs? ...changed spark plug wires, distributor cap/rotor? Based on your frustration level, you've probably done all this but I have to ask since you haven't told us what you've done besides run Datamaster.

Darkgh0st

So Far:
Replaced EGR
Replaced EGR solenoid
Checked vacuum going to solenoid (not with gauge, but I can feel strong vac)
Replaced plugs w/ NGK
Replaced plug wires with MSD 8.5mm


I havent replaced the Opti yet, but I have a feeling that it's the opti causing all of these problems. For all I know it could be the original. Would a bad opti cause knocks like this??

I've had a thread in the performance section but I haven't had alot of responses so I figured I would check here with my newly found knock info and datalog

Beth396

I looked at your posts. You have gotten quite a few responses. It appears that you don't have much history on the car. If that's the case you have to assume that the sensors have never been replaced. Anything thats relatively inexpensive and fairly easy to replace should be replaced with a car this old.

I'd replace the O2 sensor(s), MAP sensor and engine coolant temperature sensor. The ECM uses camshaft position, rpm, map, pressure and temperature to control the ignition/combustion timing. The ECM uses MAP, coolant temp. and rpm to calculate spark advance.

Your car starts in open loop and when the engine coolant temp reaches a certain temp it goes into closed loop. Closed loop uses sensor feedback to control the fuel/timing. When you're over 50% TPS (WOT) you also go into open loop mode and bypass the O2 sensor feeback. Datamaster displays open loop during warm-up.

You can troubleshoot the opti-spark/EST system to find out for sure if it's the problem. The Corvette Service manual comes in handy and is a worthwhile investment. I got mine on Ebay.

You may have to become a mechanic after all.

Darkgh0st

10 posts out of 480 views (Not including mine)

I guess that makes sense. With a car this old if it's not broke now its only a matter of time

Interesting

I've got the manuals already, I'm just not good at this stuff.

I guess I'm gonna need to keep researching

thanks for the help

tjwong

I looked at the scan data and I see only a couple degrees of actual spark retard. However there is a lot of knock counts. This isn't necessarily a bad thing because even though there is a high knock count activity it don't necessarily mean that there is a high knock retard value either. But in your case if you look closely at the data when you see 1.8 degrees of KR, the actual commanded timing drops like a rock from 24+ degrees to around 14 degrees or less.

That can cause the problems that you are experiencing. Looking at your data I don't really see anything adverse that can cause this strange problem. Does your car have some kind of custom chip in the ECM? If not PM me with the 4 letter prom code, and I will take a look at the calibration to see if there is anything in the code that can cause this. I doubt I will see anything but its worth it to take a peek at it. A knock module gone crazy can cause this but the thing is in your car the KM is part of the prom. If you got a spare prom laying around you may want to give it a try.

Darkgh0st

I am using the stock prom on THIS adapter from Craig Moates. I will try my stock PROM plugged directly and see if I still get knocking to see if its the adapter. How hard is it to replace the Knock Sensor? I've noticed that some people replace with the LT4 knock sensor because its a little less sensitive.


Thanks for the help. Much appreciated

tjwong

One other thing if that don't do it. If the PCM "thinks" that the KS is bad it will retard the timing a considerable amount. But that is usually associated with a hard DTC failure which isn't your case. But I have seen in some "rare" cases where the PCM or ECM will retard timing just as if the KS was bad but does not set a code.

Darkgh0st

Thanks for the info So my next step would be to replace the Knock Sensor with an LT4 one then??

EDIT: I guess they dont make an actual LT4 knock sensor, but an LT4 knock module for '94 and newer. There goes that idea

tjwong

Yes you can use the LT4 KM but not the LT4 knock sensor. The later 96 and up LTx PCMs used a different shunt resistor in the KS and will set a DTC if used in the earlier PCMs.

Darkgh0st

I have a few more questions if you've got a min.

-I'm under the impression that my '93 is not compatible with a knock sensor module upgrade, is this correct?
-Would bad gas or moisture cause knocking at all RPM and not just 2000RPM range?
-I would think that fuel pressure issues would more than likely cause problems at the upper RPM's and not lower, do you agree?
-I just drove the car for 3 days and it ran perfect which is really irritating because having the problem be intermittent makes it harder to diagnose.
-Could a bad opti cause knocking??
- 1 last thing, it seems that I get high RPM misfires at the same time that the knocking problem is occurring, is there a common denominator to both of these or could they be separate issues???


thanks alot for your time. I know you're a busy guy but I really appreciate the help.

miris32

I have a 93 and have been chasing a similar problem for a week or so (haven't had time to devote to it). I found a ton of fuel problems when I checked the fuel pressure and corrected them but the car still ran like crap.

Got in late tonight and pulled the distributor cap. The picture says it all. I had this cap off last summer to dry it out after my son drowned the car in a storm. I am planning an overhaul this year, so didn't really worry about putting the new distributor cap on at the time. Tomorrow I'll be installing the MSD cap w/vent kit. I'll keep you posted.

Let me know if I can help walk you through any of the mechanical stuff.

STL94LT1

Yes, only the 94-96 LTx cars can use the LT4 KM.

Darkgh0st

Let us know how the MSD works out I've had a feeling for awhile that my opti has something to do with this. Even if it's working its probably pretty beat.

miris32

The MSD worked like a charm. Got rid of all of the backfiring, missing, etc. The top of the rotor was really loose, causing the rotor to scrape the cap. I would definitely pull the Opti if I were you just to be sure. I also changed wires beacuse one had a broken boot, and I put them securely in the looms to minimize the possibility of arcing, melting, etc.

My TPS still won't go past 3.7 volts @ WOT, but I think that is related to a new accelerator pedal that I installed several weeks ago. Even with the TPS problem, the car idles very smoothly, and acceleration is pretty crisp. I have a long way to go to get it tuned the way I want it but at least it's fun to drive again. Let me know if I can help.

Purchased the Cap and Rotor from Summit - pretty good deal. I went with MSD because it's vented and I hate the moisture problems associated with the original design for pre-94 cars.

Darkgh0st

The missing that I get is identical to when I had bad wires a few years ago so I'm very confident that it's the opti.

I need to park my car for a few months to save up for a new opti. I just spent $3,500 last week on my vette & Grand Prix repairs so Im broke
I suck at any mechanic stuff so I'll have to hire someone to do my opti replacement.

Thanks for the info and the pics of your Opti . I cant wait to see what mine looks like, I'll bet its a mess.