MAF:

Check to make sure (key on) that you have 12 volts on the pink wire (pin “C”) in the MAF harness connector. Check that the black white wire (pin “B”) has a good ground. If your multimeter has a frequency scale, while cranking the engine, check the frequency between the yellow wire (pin “A”) and a chassis ground. low air flow will probably show a frequency of less than 2,000 Hz.

 

Pin B and C checked out, A will have to wait until I have a helper. Also forgot to check my meter. Went ahead and bought a new MAF anyway to have on hand.

Checked for continuity and cross-feed between the Opti wires, and those checked out good to the PCM. Also checked a ground on the PCM, and it was perfect. Have to check the rest, but I was encouraged. I was concerned about an intermittent ground issue.

Probably won’t be able to do anything else until tomorrow, and that might also be limited.

 

I suppose the ultimate question is, if everything checks out with the multimeter and scope, but DTC 16 says differently, that’s the only part that matters, right? Have to make a decision if I’m going to trade the Opti out for a different one.

 

Based on watching the battery voltage, when you turn the key to start the engine cranks very slowly, and there are RPM readings. The Opti is supplying the low resolution pulse required to start the engine and register the RPM. DTC 16 sets sometimes when there is a delay in the PCM picking up the low res pulse. In fact the code can only set when the engine is cranking.

Your ‘scope shot of the high res pulse was inconclusive, may have been the result of the scaling you applied. DTC 16 sets only when the PCM sees a certain number of high res pulses, without seeing a low res pulse.

Based on the data logs you need to focus on the MAF sensor. If it's getting 12 volts - which your test confirmed - it could be a faulty sensor. But that is the purpose of (harness connected to sensor) looking at the yellow wire for a frequency reading.

The 1997 cars had a problem with the wiring for the MAF. The 12 volt wire was breaking somewhere in the harness and causing an OBD-2 code for the MAF sensor. But 97 was the only year with that problem.

 

Fair enough. I’ll log more crank pulls tomorrow before I do anything else, and check the yellow wire.

 

We don't need more crank pull logs. Not until you solve the MAF sensor issue.

 

My multimeter does not have a frequency reading that I can see. I was hoping a log would show any MAF reading, but it does not.

Looking through wiring diagrams right now.

 

One thing that could be happening is, your scope might not be sensitive enough to see any back ground noise on the opti signal that the computer is seeing. yeah you see a good signal on the scope but, the computer maybe seeing a lot of noise along with the signal and because of that it set a dtc 16, like i said in the past there a lot of posts where the opti sensors on these chinese units are garbage!

one thing you could do is plug in your other opti to the wire harness, pull the injector fuses and turn the ignition key ON and spin the opti with a electric drill and do a computer log and see if it sets a DTC 16. also you can put your scope on it too and look at the readings on the opti and ignition system.

 

So, I have good news and I think I have bad news.

The good news is that I can confirm the Opti, spark, injectors, all work.

The bad news is I think I have a blown head gasket or head problem.

I unhooked the MAF sensor to see if I could get any result off speed density, and it kept wanting to giddy up. Playing around with the fuel delivery I got it to start. With no muffler to speak of, it was impossible to hear what the engine sounded like. Overall it was extremely loud. After about ten-fifteen seconds I shut it down and wanted my son to hear it after all the work we’ve done together.

It refused to start again, regardless of the gas pedal. When I let him take over cranking duties, I noticed a vapor coming from the passenger manifold area, gas and ever so slightly of antifreeze. It could be something else, but deep down I think I knew this was a distinct possibility. The vapor/steam/gas starts again on every crank cycle, and only on the pass side. No fluids on ground, and no fluids that I could see elsewhere. Would not be surprised if the successful crank heated things enough to bring an original problem back into scope. The attempts to start, the temp gauge kept climbing.

I will see what evidence I have on my phone that might help demonstrate, but I think the MAF sensor issue is going to have to slide down the list. I suppose there is a very small chance it could be from an exhaust leak (because at this point, what other things are going to be a surprise). But I think it is clear I need to get a compression test done next and a leak down test if at all possible. Even if I have to tow it to the shop and pay my guy. I need to know if this engine is worth putting money into.

 

The LT1 engine has a tendency to blow out the RTV sealant that seals the bottom of the intake manifold to the rear wall of the block. The RTV dries out from the heat of the corrugated EGR riser pipe plus the heat it dumps into the intake manifold at the passenger side rear corner. If you have some heavy piston ring blowby you could see some vapor coming out of that area. The LT1 intake is dry, no coolant passages, so unlikely to see coolant coming from the manifold.

Take a paper towel, reach to the passenger side rear of the block beneath the intake manifold and wipe. If the towel comes up coated in oil, you have the “LT1 intake manifold leak”. If you had coolant on top of the pistons, the rings could be sludged up and not sealing well.

 

I will check on that tomorrow, thank you. It did seem to be coming more from the exhaust (I should have been more clear) manifold than the rear, but I need to stop making assumptions. Compression is still going to be the next thing I do, I may have given up too easily. If I had a rotating collar on the end of the hose, it may have gone better.

 

Doubt that. The pcm has filtering that ignores voltage spikes while making this reading.

 

The only time I've ever seen any evidence of high voltage interference was on an MSD. The RPM in the data log showed instantaneous spikes at idle, as high as 7,000 RPM. The PCM reacted by opening the IAC (as it normally does) to keep the engine from stalling if the throttle blades suddenly closed. But the throttle blades weren’t even open. As a result of the false RPM (pulse count, not voltage) spikes and the IAC response the idle was surging from normal to as high as 2,700 RPM.

This was on ltxtech.com. There was a “tech” from MSD who was an active member. I highlighted the cause/effect in the data log and he gave it to the MSD engineers. Initially they blamed the IAC, but I was able to convince them (through the “tech” - they refused to communicate directly) that there was something wrong with the low res signal. In the 1993 ECM there is an additional column that shows the interval between the pulses.

They finally agreed it was a problem with interference, and had the owner ship them the Opti. They said they found the problem (apparently with the cap) rebuilt the owner's unit and shipped it back. The implication seemed to be there was a problem with the cap because they had changed the epoxy.. My guess was that the epoxy insulation that surrounds the metal strips that route the brass buttons on the cap to the wire towers were cross-firing (poor insulation) and the discharge was affecting the low res pulse output from the module.

At that point, I found two more cases of the problem in MSD units. MSD was replacing the units for free. Appears they had a production run with the bad caps. They would never respond directly to me, so there is obviously some speculation. And as a follow up, the unit they returned to the first owner with the problem wasn’t installed right away. When he installed it the MSD wouldn’t work at all. MSD replaced it again, but he never installed it. The only other thing that came out of this was that we became good friends, and he bought my Formula when I put it up for sale.

I don’t see any irregularities in the RPM readings in the data logs I've looked at for “It’s_Cochese!” so far. The weak battery, and the after effects of the cylinders apparently filling with coolant seems to be limiting the cranking RPM. Given the history of the engine, every single system and subsystem needs to be examined. Could easilly be disconnected or corroded connectors, damaged wiring, thing corrected wrong, etc. Really a giant 1,000-piece jig-saw puzzle.

 

I don't really know what circumstances happened to allow the engine to start yesterday, then refuse to after that. I do know that the cranking RPMs did seem to be quite a bit higher after, which is what I was hoping for. I'm going to pull a log later today simply to quantify it. Do that at the same time I look for where this vapor/etc is coming from. I think the fuel pressure kit I borrowed had a multimeter that could do frequency, so I'll get that answered.

There are some electrical issues, but these have been mostly interior ones - door locks, driver window motor, mirrors. The only thing sketch I've seen under the hood is that there is one user tie-in at the front driver side grounding point, but there's no wire there anymore - the OEM wiring is tied into an in-line connector that doesn't attach to anything, but the OEM wiring is intact. The EVAP solenoid has bare wires on the connector that is on my list to heat shrink or otherwise.

Continuity was checked to the PCM on the yellow MAF wire, and it ohmed out at basically zero. Whatever issue is there with the old and new MAF sensors, it isn't the wiring. Checking the health of the engine is temporarily the only item on my to-do list.

 

Fuel testing kit at Autozone does not come with a multimeter that can do frequency. That will have to wait until after compression test, and a leak down test after that if called for. I got down under the car and think the vapor/smoke that smells so much of gas is the result of it leaking out of the manifold junction to the Y pipe and onto the hot starter. I suppose I am lucky the connections are solid and a fire didn't start. The sweet smell corresponds with the Marvel Mystery Oil that is still on places, including a little bit on the starter.

So, temporarily, I'm not in a panic. I still have to quantify compression. I may or may not have to resort to taking the Y pipe down and the manifolds off, which I'd rather not do at this junction. I will, however, take the starter out and have it tested since it has put in so much work lately. Even though the battery is brand new, I'm sure I'll have to get that warrantied out as well before too long.

Thank you to @Injuneer @GaryDoug @chevykid for getting me to this point. May not post for a bit until I get some numbers crunched.