LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

94 LT1 Corvette owner from Sweden having Opti problem

Old 06-15-2019, 03:42 PM
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94 LT1 Corvette owner from Sweden having Opti problem

I have been communicating with Lars-Gunnar from Sweden about his Corvette's LT1 engine problems. It has the symptom of surging idle speed repeatedly while the engine is running at idle, not moving. So far I think we have narrowed it down to the vented MSD Opti that he purchased from Summit Racing 2 years ago. The scan9495 logfile shows spikes in the false indicated RPM, usually to around 3000 RPM. We have seen this symptom before in a few members LT1 cars. Interestingly, he has tried spraying cold mist into the Opti vent pipe, first away from the Opti (no changes) and then toward the Opti (major improvement). It seems that something may be losing connection and the cold spray temporarily "fixes" it. I don't remember anyone in any forums even suggesting that as a diagnostic; I certainly never thought of it. Good for him.

The attached chart shows the spikes before and after he uses the cold spray into the Opti. By the way, the spikes seem to occur even with the engine temperature fully warmed, as seen in other runs. So this isn't just an issue at the cooler engine temps.

I think he will be along soon to talk further, especially about what to use for a replacement distributor. Keep in mind that he has a significant time difference from most of us, so replies may be delayed.

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Last edited by GaryDoug; 06-15-2019 at 03:50 PM.
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Old 06-15-2019, 05:14 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

I'll take a look to see how well it matches up to what I have seen in the past with false RPM spikes from the MSD Opti’s causing erratic idle. MSD reportedly traced the problem to the cap, and claimed they changed the cap and optical module in response. I assume they were getting arcing on the cap due to problems with the dielectric strength of the epoxy they were using to cover the internal conductors that route the cap contact buttons to the correct plug wire terminal. Just a guess, though.

I'll take a look at the log tomorrow. Unfortunately my laptop crashed, and even though I had a Western Digital external backup drive, I haven’t been able to recover the files from my new laptop. WD is actively trying to solve my problem. And I may be able to recover the older data log file via the method I sent it to MSD, through a third party (they wouldn’t communicate directly with me).
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Old 06-15-2019, 05:28 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

I should mention that this version of scan9495 for the y-cars is far from prefect. There are some bad sections of most scans because of the interference with the CCM module. I could never fix that without a sample car. Usually when that happens, all the readings get majorly messed up, not just the rpm.
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Old 06-16-2019, 12:32 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

My name is Lars-Gunnar and I am new to this forum.
I have a Corvette 1994 with LT1 engine and I may have some similar problems as some of you.

Background:
I bought the car 1999, direct imported to Sweden from Michigan, USA. During the years I have had problems with hesitation and after winter storage (October to May every year) I have had faulty O2 sensors on left side a couple of times. Fault code, bad behavior which was solved with a new O2 sensor. I have also had some coolant leakage.

Autumn 2016 I decided to fix the problems. I made some research and bought parts from Summit Racing:

MSD vented Optispark. (I got the impression that it was good quality and good with ventilation)
New MSD Coil.
Cable spark plug wires
New sparkplugs
New original water pump

I mounted all parts during the winter but when I should mount the Optispark I got problem.
The machining of the spline axel had geometric faults and it was not possible to push it into the camshaft. I sent it back to Summit Racing and got another MSD Optispark and this time it fit.

During summer 2017 the engine runs better than ever except for this.
Once when I should start the engine after visiting my parents, it cranks without start. I leave the car and the next day when I should tow it home, the engine starts without any more problems 2017.

I stored the car in my garage during the winter and when I should start it in the spring 2018 I got big problem. It cranks good but did not start. I had spark, I had fuel pressure, I try to start with starter gas but it did not even try to start.
I mounted a new ICM module and new coolant temp sensor but no change.
I was a bit confused but come to the conclusion that either the spark is too weak to fire in the compression or the position of the spark is wrong. I also checked the fuel so that it was no water in it at the injector position. I build a test rig for measuring the distance the spark can fly and at sparkplug position it flies 8mm and after the coil 15 mm. It should be good enough.

How can the angle for the spark be wrong from a new MSD Optispark? The screws that hold the distributor arm in the opti could have loosened. I remove the Opti again (I also find a way to suck out the coolant without leakage but I tell you another time)
It was dry inside, look like new and the screws was in place. Very strange! I mounted the opti back again and plan to start to dig into the PCM to look for cracked solder points or electrolytic capacitors that have dried out.
For fun, I tried to start the engine before I mounted the water pump. The engine starts! Now it will be much harder to find the root cause.

I tested it for a while and it was OK except for two problems, idle rpm was surging and hesitations around 2000rpm when accelerating.
I measure the injectors. The resistance was nearly the same on every one. I have test equipment which can send a long pulse, 50x7ms pulses and 100x3.5ms pulses to one injector at a time. Without starting the engine, I pump up the fuel pressure to 42PSI,
send a pulse train to the injector, measure the remaining pressure and all injectors behaved very equal and it cannot explain the problem. I also cleaned the MAF sensor with electronic cleaner but no change in behavior. I measured the TPS voltage and it was good.

When the idle rpm starts to surge I tested to disconnect the MAF, TPS and IAC.
When I disconnect the IAC it immediately stop the surging rpm. I changed the IAC to a new but same behavior.

Now I try to find an OBD scanner to try to understand what is going on. I found the very good Scan9495 from Gary and I also got a lot of help from him.

Now it is possible to see that the new MSD Optispark disturb the PCM to suddenly read faulty high rpm which fools the PCM to increase the IAC counts. I have not checked the signals with an oscilloscope so far but I sprayed cold spray in the ventilation hose to the opti and the surging idle speed stops. I continue to log to see when the surging rpm comes back but does not. Strange! I continue to log for 20 minutes at idle but no problem.

I take the car for a ride and also the hesitation during acceleration was gone. Very strange!
I run the car to high engine temp and after that I let it continue on idle and there is no surging.
Maybe it is a crack in a solder joint or a crack in some connection inside the optical sensor and when the very cold spray hit the electronics it create a force which press the crack together?

I am thinking about next step now.
I will drive it until the problem starts again and then I will mount a new Opti?
Is the Summit Racing branded Opti good? (I dont trust MSD any more)
I have seen that it is possible to buy just the optical sensor to the original opti. Have you any experience about changing just the optical sensor in an MSD Opti? Will it fit to MSD Opti also?



Best Regards
Lars-Gunnar from Sweden

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Old 06-16-2019, 01:49 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Lars....given the many reports, and some extremely well documented and investigated (data logs) by Injuneer, the MSD Opti itself has to be at the top of the list of problem. Especially given the testing and part swaps you have done.

I too had a MSD Opti years ago and replaced more parts than I care to admit only to find the "problem" was the MSD Opti. My issue was high RPM misfire.

While I have not used one, Petris is one Opti that has been recommended and I believe Injuneer can shed more light on its merits

https://petrisenterprises.com/
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Old 06-16-2019, 01:49 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Welcome to CamaroZ28.com ! ! !

Hope we can help you out.

I did look at the data log Gary attached. I see the problem, and the interesting thing is that the erratic idle speed problem goes away early in the data log - run time 478 seconds, scan time 118 seconds, around data log record 950. Could that possibly be when you sprayed the cooling material into the Opti vent hose? I will say that when the problem is occurring, it appears to be similar to the problems I have observed with the MSD Optisparks manufactured from 2 to 4 years ago.

It is just not as severe as I have observed previously. In those cases, the RPM spikes were more frequent and more closely spaced, sometimes driving the IAC upwards to the point that the engines were idling at 3,000 RPM. In your case it appears that even with the 8 records per second logging rate, some of the spikes that cause the IAC counts to suddenly increase are not even visible. It is important to keep in mind that at the target idle speed of 750 RPM, the crankshaft is rotating 12.5 times per second, and 4 cylinders fire per rotation, yielding 50 sparks per second distributed by the Opti. That means many of the spark events are not visible when scanning at 8 frames per second.

I'll look at the comments in your post and attempt to answer your questions. I just wanted to ask the above question first.
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Old 06-16-2019, 04:37 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Yes, the time when I started to spray Cold spray into the Opti corolates very good to the time when the idle rpm stabelise.
I sprayed on and off during maybe 100 sec and then I stop and wait for the problem to come back but it did not. I attach a logfile after the one that Gary add earlyer 1000 sec more and the rpm was stable.

/Lars-Gunnar
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Old 06-17-2019, 12:09 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

I downloaded the new file, and it look like the old one, after you sprayed the cooling material. Just out of curiosity, what is that material?

Just like the first file, it shows a very stable, normal engine. If there is one thing that could be said, at idle, as the engine warms up, the PCM starts reducing the fuel to compensate for a minor rich A/F ratio. At the end of the second log, the long term fuel trims (LTFT) on both banks have dropped to 119, meaning the PCM is reducing it's calculation of the fuel required to maintain the desired 14.7:1 A/F ratio by 7%. Not a big problem, although I generally look for no more than +/- 5% as in indication of no issues with the fuel system. It could be as simple as the fuel pressure increasing pressure as the engine warms up, or the MAF sensor over-stating the mass air flow as the sensor warms up. Nothing to worry about. But it would be a good idea to scan the PCM while driving under various combinations of engine load and RPM to see what the LTFT's are for the various combinations of engine load (as measured by MAP) and RPM.

Could you please explain what the machining problem with the Opti's splined drive shaft was. I have never heard of ths problem in the 20+ years I have been helping LT1 owners n this site, and others like it.

The machining of the spline axel had geometric faults and it was not possible to push it into the camshaft. I sent it back to Summit Racing and got another MSD Optispark and this time it fit.

Please look at this reference to indicate what the machining problem was:

http://shbox.com/1/93-94_opti.jpg

Also, with regard to this comment:

How can the angle for the spark be wrong from a new MSD Optispark?
Typically, if it didn't start after you installed the new Opti, and you removed and reinstalled it and it did start, that means the splined shaft's "odd" tooth was not correctly indexed to the hole in the cam sprocket on the first install. You would be surprised to know how often this problem occurs.
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Old 06-17-2019, 03:41 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Hello again,
The cooling material that I used is ordinary Cold Spray that normally is used for finding faults in electronic hardware, see picture.
I have seen the strange low value of the IAC counts and also the low LTFT. I cant find any vacuum leakage but I will try more.

My PC battery is broken so I have to borrow another to log during driving. That will be next step.
I add a file with measurement of the faulty splineaxel. It was not possible to fit it into the camshaft with normal force. I think that some could have destroyd the bearings in the opti if they just forced it in with the mounting bolts.

I have never mounted the opti with the indexspline in wrong position. When I had the start problem 2018, the Opti has been in place since spring 2017 and during summer 2017 it runs perfect except for one time when I could not start the engine but the day after it was ok again.

Best Regards
Lars-Gunnar



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Old 06-18-2019, 12:01 AM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Low (below 128) LTFT's would not be due to a vacuum leak. A vacuum leak would cause the PCM to move the the LTFT's above 128, in order to supply the extra fuel for the vacuum leak air, that is bypassing the MAF sensor.

I'll look at the IAC counts in the second data log. In the first log I noticed a reading as low as 9, but that was something that developed as the idling engine engine just kept heat soaking. Look at the inlet air temperature (IAT). It is up to 119F at the end of the second log. The IAC counts were dropping with time, just like the LTFT's were dropping with time. I didn’t associate that with a vacuum leak.

I didn’t put a lot of time into the second log. but I will look at the IAC. The second log is about 15 minutes of idling, generating 450,000 pieces of data. Doesn't actually reflect a “normal” operating condition. To start my review, I typically try to identify the maximum, average, and minimum values for each parameter, looking for major departures from normal. I did that for the second data log, and noticed a few frames of data that were obvious data errors. Actually, there were very few. Some interference always seems to occur in these logs, no matter what software is used.
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Old 06-18-2019, 12:11 AM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Fred, you are the man. That's why I recommended he come here instead of a lesser-manned Y-car forum.
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Old 06-18-2019, 01:52 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Can the low LTFT values comes from a clogged airfilter?
I am also surprised that the IAC counts can be so low. How does the engine get air?

I have been driving around and made some logging tonight. I attach two logfiles.
Before I sprayed Cold Spray trough the Optispark a couple of days ago the car behaved bad every time, surgeing idle speed and hesitation during acceleration.
Now I can not recreate the problem. The engine runs very good.

Strange!

I also let the engine run at idle a long time when I came home and it was no surging rpm.

Thanks a lot for your help!
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Old 06-18-2019, 04:04 PM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Originally Posted by Larhed View Post
Can the low LTFT values comes from a clogged airfilter?
No. The mass air flow sensor measures the actual mass (not volume) air flow into the engine. The clogged air filter will cause loss of pressure, which reduces the density (and therefor the mass) of the air entering the engine. But the MAF sensor still accurately measures the mass of the air. Once the PCM knows the mass air flow, it calculates the required fuel, based on the target A/F ratio. Clogged filter reduces power, but the PCM still can accurately proportion fuel to air.

A question I didn't ask, but should have - are you aware of any modifications to the engine, or has the PCM possibly been reprogrammed? The target idle RPM is not a typical value for a Camaro or Firebird LT1, but may be normal for the Corvette. I just do not have that much data from the Corvette. Also, the PCM elevates the idle speed based on coolant temperature, for cold start purposes. But I think below 140-degF on a Camaro or Firebird the target RPM would be slightly higher than it is above 140-degF. Yours stays at 750 RPM through the entire log.


I am also surprised that the IAC counts can be so low. How does the engine get air?
Good question. I see from the second data log that by the end, the IAC counts are down to 1 or 2. It could be a vacuum leak, but there is no evidence of that in the MAP readings or the LTFT's. Could be the throttle body blades not fully closing. You should check the blades to make sure they are closing against the throttle stop screw. If not, could be a return spring problem. If the blades are closing against the throttle stop screw on the throttle body, try backing off the stop screw to further close the blades very slightly and very slowly, observing the IAC counts until they are in the range of 15-30 counts while the idle is holding target idle speed. When you finally get the IAC counts within range, check the TPS voltage.

Your current reading is 0.59 volts. Typical value for a stock, unmolested throttle body is 0.65 volts, but it does vary. And it in not adjustable. The PCM looks at the TPS voltage when the key hits the "run" position, and as long as the reading is between ~0.25 - 0.90 volts, it sets that reading as the 0% throttle opening, and prorates the throttle position percentage upwards from that value. As you further close the blades with the throttle stop screw, the TPS voltage will start to decrease, but as long as it doesn't drop below 0.30 volts, it will be OK. Probably will not get that low.

I have been driving around and made some logging tonight. I attach two logfiles.
I will try to take a look at them, but it might take a few days. Although I am retired, I still do consulting work, and I am preparing for a class I have to teach on Thursday.


Before I sprayed Cold Spray trough the Optispark a couple of days ago the car behaved bad every time, surgeing idle speed and hesitation during acceleration.
Now I can not recreate the problem. The engine runs very good.

Strange!

I also let the engine run at idle a long time when I came home and it was no surging rpm.

Thanks a lot for your help!
That is very good news.... but Gary and I are still trying to figure out how it "fixed" the problem.
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Old 06-22-2019, 09:07 AM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Sounds like to me it may be something in the opti that the cold spray could have moved or something he disturbed(wiring maybe) when injecting the cold spray.

Only other thing I can think of would be is the opti vent line......could it have been clogged? The Buick know how video says if the line becomes clogged, the cap can get pulled to the rotor and cause misfires. However, I doubt a blast of cold spray could unclog something that plugged. Could the opti vent line check valve been installed backwards...or not working right?
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Old 06-22-2019, 11:04 AM
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Re: 94 LT1 Corvette owner from Sweden having Opti problem

Just got around to downloading the two newest files. Only looked at the second one, but there appear to be some issues when you go to 100% throttle, it enters power enrichment (PE) mode, and Cell 15 LTFT's are in the high 140's. The lean condition appears to cause some knock retard. The EVAP system is at 100% duty cycle....possibly a vacuum leak when the EVAP purge solenoid opens???? I need to scroll through the complete logs to record the LTFT's in each of the cells you operated in to see if there is a pattern. A preliminary glance shows it's reducing fuel in low load cells and adding fuel in high load cells.
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