Please Help with my 5.7 Vortec!

Hello everybody. I’m new to this car community thing but I am at a dead end everywhere I turn with this truck. Please bear with me guys as I will go into great detail to ensure you all can try and find the problem; I will try and keep it short. Last Friday, I jumped on a 1997 Tahoe 2x4 with the 5.7 Vortec. The guy only wanted 1400$ so I couldn’t refuse. It has 334000 on the motor and 60000 on the rebuilt transmission which is stock. Perfect project truck. I immediately noted a very slow start into a very rough idle at about 500rpms. The previous owner stated when he rebuilt the transmission, he also replaced the distributor cap and rotor and that it would be wise to replace the entire distributor in which I did. With a little bit of fine tuning of the timing I was idling smoother but still missing on cylinder 5. I replaced all the plugs and wires and still no change. I cleaned the cold air intake filter before all of this and oiled. Now, for some reason that I can’t understand, the truck will crank but never start. It’s actually blowing air and fire out of the throttle body as soon as it should be starting. It’s actually trying to push the filter out of the box sometimes. I guess that’s called a backfire? The only way it will crank and start is if somebody holds the butterfly valve open on the throttle body and even then it is a very rough idle and as soon as I put it into gear, it stalls. When I bought it, it was showing a P0300 code which is the random misfire lean/rich code. I just don’t understand how it ran before and when I put intake manifold back on, everything is different and inoperable. Please help me out guys, I am welcome to any constructive criticism.

Hopefully you didn’t use to much oil and ruin your MAF sensor.

1 Like

If the intake manifold was off, you had to remove the distributor, which leads me to think…

You installed the distributor 180 degrees out of time, I think.

Remove the first sparkplug on the drivers side. Mark the location of the wire for that plug on the distributor body. Remove the cap so you can see the rotor. Rotate the engine by bumping the starter or with a wrench so the the rotor points to that cylinder as you have your finger stuck in tbe plug hole. If pressure doesn’t build up, you aren’t in the right spot!

6 Likes

You say it ran before. That leads to the following questions. How well did it run and how do you know?

At 334k miles a compression test is mandatory and I suspect the engine is worn out.

4 Likes

If it’s back-firing out the intake, the timing chain may be stretched or jumped time.

Tester

1 Like

I had my friends dad do this to ensure I was at TDC and on compression stroke and it was. Point right at cylinder 1

I know because I drove it to the shop 30 miles away and it drove really good. Yes, rough idle but at highway speeds it had excellent power and response time.

I also just found out that my friend that did research on spark plug gap said .035 for 5.7 vortec and it says .060 in the manual. Trusting him, I gapped them all to .035 and installed them… Could this be the reason I’m not starting, back firing from throttle body and when I do it stalls while in gear with a very rough idle?

No, .035 gap should b fine… based on what you told me about finding TDC and the mileage on the engine, I agree with @Tester, sounds like the timing chain jumped.

So how sure are you that some plug wires are not crossed up? Or maybe left a PCV hose disconnected since you mention intake off.

If the wires are correct the engine still has a problem with No. 5. That could be a compression issue or even something like a weak valve spring on that cylinder.

I disagree. .035 is way off from .060 there’s no way timing chain was good before I changed the distributor and now all the sudden its bad. That doesn’t make sense.

Reduced spark plug gap won’t prevent the engjne from starting or even make it run badly. But then you came here for advice, what do we know?

And a timing chain can jump a tooth or more in an instant. One moment ok, the next not. Good Luck, I am sure you can figure this out on your own.

I’m seeing .060 as the correct spark plug gap spec for the 1997 Tahoe V8-5.7L VIN R engine. Suggest you research this more before assuming the smaller gap you are using now isn’t the cause of the no start. If the gap isn’t the cause it sound like some sort of timing problem. Could be the valve timing (i e timing chain) or the spark timing (distributor).

This post was flagged by the community and is temporarily hidden.

It went from running with a miss on #5, to not starting at all and backfiring, you did nothing between these 2 things ?

My guess’s are distributor moved or timing chain jumped.

Or new distributor is no good.

2 Likes

Your comment to Mustangman is way out of line and whether you like it or not, he is correct. The plug gap is NOT going to cause this kind of problem.

You say the timing chain is good and there is no way it can go from good to bad just like that. Yes it can; including during one simple revolution of the engine without it even being started. You think a 334k miles chain setup is infallible?

5 Likes

I don’t have much to offer other than if it runs worse now than it did before you started working on it, my first step would be to check my work. Check plug routing, check timing (maybe even put the old distributor back in), make sure the MAF is clean, make sure you don’t have a gross vacuum leak, since the manifold was off. It definitely could’ve jumped time, but I’d lean more towards a mistake being made since you just worked on it.

2 Likes