Wont start looking for ideas cause I really have none

1997 CHEVROLET TAHOE
3,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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I spent 2.5 yrs rebuilding this truck. New motor, transmission, transfer case ect. Lets shorten the list and say a solid 90% of mechanical and electronics are all new. Im getting ready to tear into this after it being down about 6 moths and Im looking for ideas cause I really have none. Nothing makes sense.

History

I put it on the road. Drove it daily for about 3 months and ran perfect. One day I pulled into a parking place and shut it off. Did my thing, got back in and started it, backed out of the space, put it in drive. All was normal. I drove 50 feet and the motor popped out the throttle body once and wanted to shut down. I put my foot to the floor and it stayed running but acted like it was running on 4 cylinders. Let my foot off and it shut down. Never to start again.

Symptoms

Crank it over and it loads out every revolution in the same place. For a split second sometimes it will fire enough to kick the starter out and then the starter slams right back in. For the older mechanics the symptom is exactly like the timing is over advanced just to the edge of not starting. If I were to hear it cranking on a 1970s car I would say the timing was way over advanced.

Work done

Since the symptom seems to be timing related I have replaced,

Crank sensor
Cam sensor
All 4 O2 sensors
Ignition module
Map sensor
Cleaned MAF sensor and tried a back up

Tests done

1 Checked fuel pressure, right on spec.

2 Checked for pinched or chafed harness

3 Tested spark

4 Compression test 145 to 170 with in 15% and only has about 3000 miles on new motor.
Thought maybe timing chain or bent valve issue but compression is perfect.

5 Tried different PCM, does the same thing

6 Checked all fuses

7 Checked all grounds

8 Pulled distributor to check teeth and gear pin and visually confirmed its rotating normal.

9 Tried starting fluid while cranking to see if the new MPI conversion went bad but any fuel at all added instantly floods it.

I thought maybe the cam for the crank sensor may have twisted but I cant wrap my head around that cause it on a key and sandwiched behind the harmonic. Grasping at straws.

At this point My plan is to get a timing light and see what the actual timing is when cranking and pull the whole harness and check every wire for continuity end to end from the PCM. My gut is saying its something simple and Im not seeing it.

An ideas would be awesome cause this is blowing my mind. Everything tested says this thing should run. Thanx!
Jun 12, 2021 at 12:35 PM
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ASEMASTER6371
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When you pulled the distributor, did you make sure the gear is not bad? It should be thick. When they wear out, the edges are razor sharp.

The cam retard on your scan tool should be 0 plus or minus 2 degrees.

The timing light will not give you the actual timing.

Roy
Jun 12, 2021 at 1:32 PM
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DANNY L
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See where the timing is at... If I remember my K1500 5.7L had to be 0 degrees when idling. I had to use a GM Tech 2 scan tool to program...
Jun 12, 2021 at 2:04 PM
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HMAC300
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i'd check the dist gear like previous thread take cap off and spin motor you'll know right away cause it won't spin right. Gm had a lot of trouble with that due to a roller cam and it would eat the gear up. i'd check that first.
Jun 12, 2021 at 2:48 PM
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94 TRANSAM
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As I said above. I pulled the (new) distributor and checked the gears and pin. All are perfect and it is a new all metal dist. not the junk stock plastic one.

And you can check the timing with a digital light. You watch the mark on the chain cover with the light while its cranking and turn the dial on the light till the ballancer mark lines up with the mark on the TC cover and then read the scale on the light dial and that will tell exactly where the timing is motor running or not.

I cant set or check the cam retard until its running cause you need 1500 rpm to set that.
Jun 12, 2021 at 6:16 PM
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94 TRANSAM
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See Im lost on this one cause its for sure acting over advanced and the PCM controls that. Its not the PCM cause my spare does the same thing. So what can give you symptoms of over being advanced if the timing is right?
I thought cam timing but the compression is perfect in all 8 and if the cam was off Id have compression bleed and low compression.

So what give those symptoms if its not the timing? I have checked everything I can think of.
Jun 12, 2021 at 6:21 PM
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94 TRANSAM
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Ok so I just checked the timing and it is 0 degrees when cranking so the over advanced symptom is not timing and if its at 0 that means all sensors are working so it has to be mechanical.
The only thing mechanical would be the cam. What are the odds a new timing chain jumped a tooth or 2? And would only 1 or 2 teeth jumping effect a compression test?

I have 170 in 4 cylinders. Does anyone know what the psi should be for a fresh 350? Maybe it is low from bleed and it could be like 185? I dont remember what full compression reads at 9 to1.
Jun 12, 2021 at 7:07 PM
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HMAC300
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a different pcm will not work until you reset the security system to that pcm i think what has happened is your security system engaged and shut the fuel off like gm products of that era do.put the old pcm in and try to reset the security system if not you may need it reprogrammed for your security system.
Jun 13, 2021 at 6:15 AM
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That was a good thought and thank you but there is no security at all in the 97s and I know its getting fuel and spark.
Jun 13, 2021 at 8:52 AM
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STEVE W.
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170 sounds a bit low for a fresh 9 to 1 5.7, I'd expect 180-190 on one with good rings like that. If it's the cam I would look at the crank key first, I can't see a new chain and sprockets jumping that soon unless something real odd happened, but I have seen a crank key shear and cause issues, although that usually caused retarded timing due to the direction the sprocket rotates. Do you have a way to read the cam sensor signal or cam crank correlation with a scope? That would tell you instantly if the cam has moved.
Jun 14, 2021 at 12:11 PM
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94 TRANSAM
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Good call Steve, but help me here with the math because something doesn't make sense. Let's say the crank key did sheer. The crank sensor cam rides on that same key so in my mind that would be as out of time and even out of time the same as the cam would if they spun equally right? Which would put all the sensors in time and just the crank out of time which means the PCM would have no idea where the crank actually is right? So how can the last 2 tests tell me anything? I have never had a key sheer on anything less than like 700 horse and not PCM controlled. Just a learning question. Is it a situation of if the key sheers the crank cam will always go out of time with the cam?

As for checking those I wIll have to see if those test are available on my scanner for this truck. The tests get weak in my scanner for cars this old.
Jun 14, 2021 at 6:33 PM
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STEVE W.
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The crank sensor is timed off the front key, the cam drive sprocket off the rear key. One could shear without the other failing. The issue then would be why did that key fail? The usual reason is that the crank bolt wasn't tight enough. The key isn't supposed to bear all of the load, it shares it with the bolt clamping the parts together on the crank snout.
End result is cam and crank out of time with each other. Same thing that sets a cam crank code on the newer cars when a chain stretches or a cam phaser fails.
That's why I suggest the scope, If you scoped the cam signal, the crank signal and used Cyl 1 as the trigger it would show the various sensor positions relative to each other without needing to open the engine. Beyond that it should also have a default value it substitutes in if it gets a bad cam signal input. That is how you can get a cam code but the engine still runs, however in this case it's out of time and doesn't run.
Jun 14, 2021 at 7:43 PM
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94 TRANSAM
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Thanks brother Ill check it out and let you know what I find. I didnt recall there being 2 keys.
Jun 14, 2021 at 8:34 PM
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HARRY P
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Ok so I'm dipping my toes back into the 2cp game. What we know is that we have (1) fuel (2) compression (3) air and (4) spark. If that's the case, then the only logical conclusion left is timing. Time to pull the cover off and see what's going on in there.

Had a similar thing happen to my wife's car last year. Didn't have the time and energy to do it myself, so I took it to a buddy to put a new timing set in it. It's a 2006 Saturn Vue with a 3.5 V6 (the Honda engine). Got it back. Left his place and it was fine. Got about 10 minutes down the road and I opened up the throttle to pass someone (they were going slow and I don't really do slow). By the time I got home it sounded like someone had put a bigger cam in it or a higher flow exhaust. It was kinda mean sounding. Ran ok too. But it was throwing 6 misfire codes and 2 system too lean codes. Dug into it and checked everything that wasn't in the timing and found nothing. We dug in together and found that the bracket for the tensioner had got stuck and both sides jumped. The odd side went forward a tooth and the even side went backward. Surprisingly the car still ran and no valves were bent, so we replaced the tensioner and bracket again and reset the timing. It's been fine since then.

Basically, if it looks like a duck and it quacks like a duck, it's a duck. It has to be in the timing.
Jun 19, 2021 at 7:17 PM
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Ya I knew it was but I couldnt find anything wrong and then Steve made his post and the next day it all came together for me. Its a fresh motor but when I did the transmission I put a hard shift kit in it. That slam shift I know is hard on the motor and as I said in my story all was well until I drove 50 feet and it occurred to me thats when it would have shifted and that was the last shift the key could take. I spent the day disconnecting everything on the motor so tomorrow I can just pull it out and then Ill know for sure. Thanks guys Ill keep you posted. Now I have to figure out how to keep it from happening again!
Jun 19, 2021 at 11:14 PM
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Ok so I yanked the motor, dropped the pan and opened the timing chain cover and no sheered key and the timing marks are right on. At this point Im thinking 5 gallons of gas poured over the whole thing will fix my issue. Well its out so ill beak the whole thing down and see what I find out. Stay tuned. Thanks for all the feedback.
Jun 20, 2021 at 5:58 PM
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DANNY L
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This might help.... Things could be worse...
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Jun 20, 2021 at 5:59 PM
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94 TRANSAM
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True, at least after 2.5 yrs work my truck will roll so at least I can take it out for a push down the road.
Jun 20, 2021 at 6:05 PM
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DANNY L
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Any updates for us?
Jun 23, 2021 at 6:19 PM
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94 TRANSAM
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So no updates yet cause I dont have time to touch it till the weekend but I can share what I know for sure and you all can add it to your data banks for the future though I think only Im lucky enough to see this weird stuff.

Symptoms,

Belching out the top on every revolution then cranking.
Wont start
Acts like timing is over advanced when cranking.

What I know it wrong thus far. Full report this weekend.

Motor is moderately hard to turn over by hand and has one spot that is harder in the revolution.
I can hear metal grinding when I turn it by hand

There are no metal shavings in the pan so I dont know what could be grinding.

Since I have the one hard spot and it pops once every rev when cranking I suspect the issue is in 1 cylinder.

Thinking maybe a roller lifter on an exhaust valve seized or fell apart which would make a hard spot on a revolution when it tries to climb the lobe. Also could explain the popping out the top cause if the exhaust valve doesnt open it cant bleed off compression and would blow back out the intake valve when it opened. Plus the fuel may still be getting ignited in that cylinder in which that pressure would remain until the intake opened.

I dont think it is an issue with any bearings cause that wouldnt effect timing and would still start.

So thats where Im at. Feel free to speculate with me. Maybe we could start a pool and see who was right LOL.
Jun 23, 2021 at 9:10 PM
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DANNY L
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Haha! ok... Let us know after the weekend what you find..
Jun 23, 2021 at 9:28 PM
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94 TRANSAM
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Ok so Im more lost now than when I began. Im thinking exorcism now. There is absolutely nothing wrong in the motor. I have no idea what the grinding was I guess the hard spot was compression cause the plugs were in. I yanked the plugs and it spins quiet and easy now so I have no idea.
I still cant see how a harness issue would cause this.
So tomorrow Im going to rip the harness open and check every wire for end to end continuity. If I dont find anything then I give up and ill just dump it at the dealer.
Jun 26, 2021 at 9:43 PM
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DANNY L
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Burn it with fire!! Hahahahahaha Kidding. I love 88-98 OBS Chevy's. I've owned 2.
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Jun 30, 2021 at 10:38 PM
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94 TRANSAM
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Hey guys I dont have access to our 2 resources for info. Can someone look up a 97 tahoe 5.7 4x4 auto and tell me were the 2 grounds on the top center plug on the PCM go to.
Jul 10, 2021 at 11:56 AM
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STEVE W.
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What color connector? There are a few for the PCM. C1 (blue) pin 23 and C3 (white) pin 17 are black/white wire and goes to the thermo housing stud.
C3 (white) pin 18 (solid black wire) is a primary ground that runs through the system and also connects to pin 12 in C4 (black) and pin 26 in C2 (red).
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Jul 11, 2021 at 7:33 AM
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Thanks Steve, Yes I have a blue, red, white and black but there is a 5th connector that has 5 pins and 3 are empty. It just has 2 black wires coming off of it. I need to know where they go.
Jul 11, 2021 at 9:22 AM
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STEVE W.
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Okay, that is C5 and those wires are just a loop. They are listed as "Service Power" which is something you plug into with the PCM on a bench. Not a ground wire.
Jul 11, 2021 at 11:35 AM
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94 TRANSAM
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Thanks brother
Jul 11, 2021 at 4:57 PM
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94 TRANSAM
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Ok so unless Im stupid this is a nightmare. Need some help tracing this cause I dont have access yet to our sources.

Cam, crank, MAF etc all have pink wires which I assume is the reference voltage feed. All my pink wires in the harness read 38.8 ohms to the black and white harness ground everywhere. Also I have 118 ohms between all the green and white and pink. Traced the green and white back to the relays in the fuse box under the hood. Dont know if I should have a reading between them but I know I shouldnt have one pink to black.

Can anyone tell me if the pink to green is normal?
Jul 15, 2021 at 12:10 PM
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STEVE W.
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That is going to be impossible to say as the Pink wire should be battery voltage feed to all those items and is fed from multiple fuses. ECM-1 and ENG-1 cover most of the powertrain feeds. However the resistance is going to vary depending on what is on the circuits.
ENG-1 feeds the EGR, purge valve, cam sensor, O2 sensors, MAF as well as a feed to the ECM.
ECM-1 feeds crank sensor, injectors, ignition coil and driver module, and a feed for the ECM.
To test them you would be better off pulling the connections and testing to see if the 12 volt feed changes. For instance if you test it and the battery voltage is at 12.6 volts but testing the pink wire shows 10.6 then you have a problem, pull the connectors off each part in the circuit until the full voltage returns, then test the part that you pulled last.
Jul 15, 2021 at 5:14 PM
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94 TRANSAM
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Nothing is connected. No battery, motor is out and the whole harness is cut open to trace wires. So Im not testing voltage Im testing continuity from each wire at the sensor plug to each pin on the PCM plugs. Thats how I found continuity from the pink on every sensor to 5 other wires at the harness connectors.

So thus my confusion cause as you listed ENG 1 and ECM 1 run different sensors but both those circuits test 38.8 ohms to ground. Which is off the planet odd that 2 circuits test the same to ground. And there is no reason I should have any reading at all to ground when neither end of the harness is plugged into anything. And I can find any melted wires. Blowing my mind.

I literally traced the pink from one end to the other and found nothing wrong so how is it finding a path to the black/white? There has to be a branch that goes through the firewall or something.
Jul 15, 2021 at 9:54 PM