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For two years now my Camaro Z28 1997 got a strange issue. I was crusing around and suddently engine turned off (like out of gas) and tach meter stopped working. I pumped the gas pedal and the engine started again but tach was still at 0, after about 20 minutes drive the tach started working again. It only happened at this time, but later it have been come more regular even if not happens every time during drive. No SES light, no error codes on OBD. I only drive the car in summer time at sunny weather.
After a lot reacher I i found that there seems to be some more with same issue but no solution. I will now start to find the problem and have an idea that the wiring from Opti to PCM can be the issue. But - and correct me if I'm wrong - if this cable have any kind of problem how come the car gets running again efter a few second but not the tach? Does anyone have solved same problem?
If the engine is running, the required low resolution pulse is being received by the PCM from the Opti. If the low resolution pulse signal from the Opti is lost, the PCM sets code P1371. I don’t believe that code turns on the “SES” light. I know it doesn’t in OBD-1, not sure about OBD-2. Causes of intermittent loss of signal can include corroded Opti harness connectors, or damaged harness wiring between the Opti and the PCM. Or, as a result of heat soak causing the Optical module in the Opti to stop producing the signal. The latter is not unusual with most replacement distributors. Do you know what brand distributor is on the engine?
I need to look at the 1996 factory service manual (virtually identical to 1997) to check the codes.
But, the above does not explain the engine running and the tach not working. I need to look at the wiring diagram to see exactly how the RPM signal gets from the PCM to the tachometer. Right now I'm limited by time on the site due to a death in my immediate family. Unfortunately, there are not many active members on this site. Maybe 6 LT1 owners who come here occasionally. I seem to end up replying to most of the questions. I do this on two other LT1 oriented sites. In general, websites like this have declined in participation. Everyone tells me that all the action is now on Facebook, but on the rare occasions I have looked, some of the stuff being posted there is totally incorrect.
If you don’t have a copy of the factory service manual, there is a free download here, courtesy of GaryDoug:
[QUOTE=Injuneer;7017980] Thank you for kind answer and help. Sorry for your loss. I don't know the brand of my opti (no brand printed on the front or back). I replaced it seven years ago then I restored engine. I will check the wiring from opti to pcm if there is any wiring/connection issues. I got the service manuals (both digital and paper). My best guess for now is that one of the four cables or connections from opti to pcm is for the tach and worn and therefore engine works but tach doesn't (the tach problem always start then engine begins to behave as it's low on gas). Just a guess so will see how it turns out. Since there is no fault code it's hard to track down errors. I think this forum and also others are greater that Facebook. During years the forums is a great source for help and is possible to find all posts. Thanks for keep the forums running! Let me know if you come across anything.
No. There is no specific wire from the Opti for the tachometer. The optical module in the Opti is essentially a (pseudo) crankshaft position sensor and a camshaft position sensor.
As far as the 4 wires:
Pin A - red/black - PCM supplies 5volt reference signal to optical module. LOW resolution side of optical module pulls that circuit down to 0volt to create a 0-5volt square wave pulse signal. There are 8 variable width pulses that the PCM uses to see which cylinder is approaching top dead center.
Pin B - purple/white - PCM supplies 5volt reference signal to optical module. HIGH resolution side of optical module pulls that circuit down to 0volt to create a 0-5volt square wave pulse signal. There are 360 1° slots the allow the PCM to precisely monitor crankshaft position based on camshaft position. In effect, the PCM knows within 1° resolution exactly where the crankshaft is in its rotation. This significantly reduces “spark scatter”, producing extremely accurate timing of each cylinder's injector and spark plug. This feature allows the LT1 programming to use a very aggressive spark timing table, improving power, and reducing emissions.
Pin C - red - 12volt power supply from the PCM to the optical module
Pin D - pink/black - low reference (ground side) side of power circuit from PCM to optical module
The PCM calculates when to fire each plug and injector, while converting the low resolution pulse rate to RPM, and sending a pulsed RPM signal to the tachometer. You need to find which wire runs from the PCM to the instrument cluster, and also look to see if there are any “modules” in the cluster in between the wire that connects to the cluster and the tachometer.
References in the factory service manual (I only have 96, but 97 should be similar):
Schematic - page 8A-81-0. Note the "solid state logic" chip in the circuit for the tachometer. Maybe subject to heat soak??? Problems with the 12 volt power supply would affect both the tach and speedometer. Problems with the common ground would affect the entire panel. Wire from the PCM connector C1 pin 13 to the cluster is a potential problem. Then everything is dependent on the internal construction of the cluster.
Troubleshooting - page 8A-81-22. Chart #10. Looks basic, checking the wiring, and requires a Tech-2 tool to run the accuracy check.
This is not a problem I have seen frequently. Sort of rare in the 25+ years I've been doing this. If you were in the US, I would say try to find a replacement instrument cluster just to test in your car. But I doubt that's practical in Sweden.
Once again thank you for your kind help, Injuneer! As you wrote it's not that easy to find a replacement instrument cluster in Sweden I don't have any other problems with the instrument cluster and speedometer works fine so perhaps it's a heat soak problem but in the PCM? I had a issue some years ago with the BCM with poor soldering (gave wired problems like power windows didn't work from time to time, radio same problem etc). Else it may be a wiring problem - I have smelled coolant sometimes and have an idea that there is a very small leak close to water pump that during hot summer days leak misty that short circuits and then the heat makes the engine and tach work again. I will start to check the wiring from Opti to PCM. I have a Tech-2 and other OBD-tools also if need, but so far no error codes. I will read the manual on suggested pages as well.
Faults in the PCM are extremely rare. But so is the problem you are trying to solve. The BCM fails frequently, particularly the solder joints, never heard of that problem in the PCM. But there's always a first time. If it was only the tachometer, the cluster would be highly likely. But the engine shutting completely down at times complicates things. I wish I could come up with a better analysis.
I agree. It's a tricky issue since it always starts with engine behavior like running out of gas (even if it's full) and at the same time tach goes down to zero. There are some other in the forums that had same problem but no solution is mentioned in thoose treads. Will let you know after checking wirings etc this weekend. Also will replace with new coil and ICM this week.
Tonight I finally dug further into the old distributor and took it apart. Looks like it has been renovated. I bought it from a company in Sweden as new but now I can see that silicone has been added as a seal instead of a gasket. As you can see it is past its best before date and not sealed. Measured with a vacuum pump and of course there was no vacuum. There seems to have been liquid in it even though it was now dry (it must have dried out during all the months it was in the warm garage). However, alkso one of the two screws for the rotor had fallen off and was inside the distributor. The rotor looked quite worn also. Finally, I also measured the vacuum on the new distributor that I installed and it didn't give any vacuum either! (bought at Rockauto). Luckily, I haven't installed a water pump etc. so I will have to take the new distributor down and check it as well. This in brief. For the time being, the theory that the leaky distributor has absorbed moisture from the leaking seal and may have caused the disturbances probably holds.
How are you testing for vacuum containment? The Opti operates under less than full manifold vacuum, due to the flow restrictor and check valve in the line to the intake manifold. If the Opti experiences high vacuum, the cap can be sucked inward and damage things. High vacuum might pull oil in from the timing cover.
Check the seal around the harness connector. That can be a vacuum leak, or allow water/coolant to enter. Also check the pins in the harness connector for corrosion. In the photos it appears there is a lot off corrosion. That might indicate the vacuum system, designed to remove the corrosive ozone generated by the high voltage discharge, was not working correctly.
Generally a good idea to apply Loctite thread locker compound to the rotor screws.
Today I tested the new opti by using a hand pump breafly pumping but got no pressure at all, nothing. Just a sound of air. Made it reverse and blow a very small amount of air into the opti and with a leak seaking solution noticed that are two places that leak - at the connection and backside. Hm, this is an new opti and can't be OK?!
Did you plug the air inlet, before trying to pull a vacuum on the air outlet? I suspect you did, but had to ask the question. Are the cap and base held firmly together?
Not sure about testing positive pressure containment. The system is not designed to operate under pressure. It is just intended to pull a small volume of air out of the corrugated rubber 90° inlet elbow (hence, filtered, MAF measured air at ~ atmospheric pressure =0”Hg).
The driving force is intake manifold vacuum, ~ -20”Hg at idle, dropping close to 0” Hg at WOT. As a result of the system pressure difference creating the air flow, there is a pressure profile ranging from 0’Hg on the pressure end and -20” Hg on the suction side. The Opti is roughly in the middle. The (negative) pressure profile is created by friction in the system. There are more elements creating friction on the suction line - the check valve and the flow restrictor orifice/filter, so it's safe to say under most operating conditions the pressure difference at the Opti never exceeds -10”Hg, and sees vacuum that high primarily at idle. Deceleration intake manifold vacuum can exceed -20”Hg.
I honestly have never measured vacuum at the Opti, so I have to stick with the above guess. We did have one person who was concerned because when he applied full vacuum to the Opti, there was a visible leak of oil from the back. Tried to explain why that was not a reasonable test, but couldn’t convince him.
You appear to be indicating you saw no appreciable vacuum during your vacuum test, and (also ??) no appreciable pressure during the pressure test. Is that correct?
The pressure leak at the harness connector could be a sign of a worn seal on the harness side of the connector. As mentioned earlier, that is a known source of leakage under vacuum.
Did the Opti from Rock Auto have a “brand” name? Most Opti's on the market are poor quality Chinese rip-offs. The consensus seems to be only a new Petris or a GM/AC Delco rebuilt are reliable.
"Did you plug the air inlet, before trying to pull a vacuum on the air outlet?" Yes. Also checked the vacuum nozzles and no problem there.
Yes, I have used about 10 Hg then tested vacuum and also then adding compressed air into the opti. But still get leaks.
Not checked the check vavle and flow restrictor yet since the opti not is mounted to the car, will do that after installation. First I want to solve the opti leaks.
"You appear to be indicating you saw no appreciable vacuum during your vacuum test, and (also ??) no appreciable pressure during the pressure test. Is that correct?" Correct. No vaccum at all and bubbles from leak seaking fluid appear then blowing a small amount of air into the opti.
"The pressure leak at the harness connector could be a sign of a worn seal on the harness side of the connector" It's new harness with new seal but still some leak. I have sealed with silicon and shrinking tube - no leak after that.
"Did the Opti from Rock Auto have a “brand” name? It's a CARDONE841833H bought from RockAuto.
Since last time i wrote I have triead to seal the leak at the opti with no success. Any ideas is welcome:
The using compressed air I have added about 0.5 PSI that will be 1 in Hg if I have done my math correct. With vacuum test no vacuum at all is created.
New opti a Cardon bought from Rockauto, leaking then adding a very small amount of compress air (0,5 PSI) with leak seaking fluid.
Test 1 was to try to seal the leaking parts with heat/cemical resistent silicon. Didn't work due to the silicon was to hard to add in the small gap (about 1 mm) made the rotating part also got some silicon and was stuck.
Test 2. After removing the silicon I tried to spray heat resistance engine color in several layers. Made it easier to get into the small gap but didn't stopped the leak.
Sealed the connection with silicon and shrinking tube.
For now I'm out of ideas since the is not possible to seal from inside either. There is two options, acccept the leak and mount opti as is or buy an new better one. No, idea what brand that is OK? Suggesttions are welcome. From what I read all brands are more or less bad. Best is to rebuild a original, I don't think there is anyone rebuilding these longer. Guess Dr opti doesn't do it longer.
Your original problem was the engine shutting off and the tach dropping to “0”. I don’t see how this can possibly be related to the Opti not holding vacuum or pressure. And I don’t feel the pressure test is even meaningful. The seals are not designed to contain pressure.
It is also very hard for me to understand how it is even possible for a reasonably new Opti, with an intact seal between the tightly clamped Opti cap and body, and a plugged air inlet nipple will not create/support any vacuum at all. But, I have never done such a test.
You indicate you have seen reports of this problem elsewhere. Do you mean the engine/tach shutting off, only the engine restarting? I don't recall this being a common problem. Or do you mean the problem with the vacuum?
You indicate you believe this forum is possibly superior to Facebook. But there is no one left here any more. Apparently they have all migrated to Facebook, or so they tell me. I'm basically the only person left on this site. There are maybe a 1/2-dozen others who drop in from time-to-time to help. But after many weeks, no one else has jumped into this thread. I have been here for 25+ years, and end up answering 95% of the LT1 questions. But this one has me puzzled. If I still owned my 1994 Formula (which was converted to a vented Opti 24 years ago) I would get out my Mighty Vac and see if I could pull a vacuum. But I cannot. I suspect it should be able to build a few inches Hg, while the vacuum pump is being operated, and that whatever vacuum is developed would drop slowly. But that's just a guess.
I would not recommend buying a new Opti because the one you are testing will not develop nor hold any vacuum at all.
”Shot in the dark” - try to contact tech support at Petris Enterprises. Ask them about the problem with the engine shutting off. Ask them if the Opti is supposed to hold any appreciable amount of vacuum with suction on the air outlet, and the air inlet plugged. Hint that the answers would help you decide to buy a new Opti, possibly from them.
03-05-2024, 07:45 AM
I don't have any other problems with the instrument cluster and speedometer works fine so perhaps it's a heat soak problem but in the PCM? I had a issue some years ago with the BCM with poor soldering (gave wired problems like power windows didn't work from time to time, radio same problem etc). Else it may be a wiring problem - I have smelled coolant sometimes and have an idea that there is a very small leak close to water pump that during hot summer days leak misty that short circuits and then the heat makes the engine and tach work again. I will start to check the wiring from Opti to PCM. I have a Tech-2 and other OBD-tools also if need, but so far no error codes. I will read the manual on suggested pages as well.
