Wont start looking for ideas cause I really have none

94 TRANSAM

MECHANIC
1997 CHEVROLET TAHOE
5.7L
V8
4WD
AUTOMATIC
3,000 MILES

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!

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Saturday, June 12th, 2021 AT 12:35 PM

30 Replies

ASEMASTER6371

MECHANIC
52,797 POSTS

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

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Saturday, June 12th, 2021 AT 1:32 PM

DANNY L

MECHANIC
5,648 POSTS

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.

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Saturday, June 12th, 2021 AT 2:04 PM

HMAC300

MECHANIC
113 POSTS

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.

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Saturday, June 12th, 2021 AT 2:48 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Saturday, June 12th, 2021 AT 6:16 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Saturday, June 12th, 2021 AT 6:21 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Saturday, June 12th, 2021 AT 7:07 PM

HMAC300

MECHANIC
113 POSTS

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.

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Sunday, June 13th, 2021 AT 6:15 AM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Sunday, June 13th, 2021 AT 8:52 AM

STEVE W.

MECHANIC
13,035 POSTS

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.

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Monday, June 14th, 2021 AT 12:11 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Monday, June 14th, 2021 AT 6:33 PM

STEVE W.

MECHANIC
13,035 POSTS

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.

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Monday, June 14th, 2021 AT 7:43 PM

94 TRANSAM

MECHANIC
680 POSTS

Thanks brother Ill check it out and let you know what I find. I didnt recall there being 2 keys.

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Monday, June 14th, 2021 AT 8:34 PM

HARRY P

MECHANIC
2,292 POSTS

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.

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Saturday, June 19th, 2021 AT 7:17 PM

94 TRANSAM

MECHANIC
680 POSTS

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!

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Saturday, June 19th, 2021 AT 11:14 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Sunday, June 20th, 2021 AT 5:58 PM

DANNY L

MECHANIC
5,648 POSTS

This might help. Things could be worse.

Images (Click to make bigger)

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Sunday, June 20th, 2021 AT 5:59 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Sunday, June 20th, 2021 AT 6:05 PM

DANNY L

MECHANIC
5,648 POSTS

Any updates for us?

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Wednesday, June 23rd, 2021 AT 6:19 PM

94 TRANSAM

MECHANIC
680 POSTS

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.

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Wednesday, June 23rd, 2021 AT 9:10 PM

DANNY L