I'm out of ideas so I thought I would ask the experts here. It's not about a Corvette, but our 1989 TBI full size Blazer.

We gave the 350 to the machine shop, got it back as a 383. Edelbrock heads, comp cam and a better intake.

At first it ran great, NOTHING HAS CHANGED, but now it's backfiring and wants to die all the time and has NO power. We have replaced the distributor, Electronic Spark Timing module, new computer and PROM chip, knock sensor and street damper (the timing marks can't move). We have quadripple checked timing. We've even advanced it and retarded it one tooth.

The computer wants you to unplug the "set timing" connector, engine warm....... and set it on 0, then reconnect the connector and the computer will take over. Now with it set at 0, it backfires and has absolutely no power. If I disconnect the connector and set the timing on 15-17, it will run, without backfiring, but doesn't have the power it should. BUT STILL, setting it at 15-17 is WAY higher than it's supposed to be and it's still not running right.

Suggestions?

 

What does the sticker in the engine compartment say? I would think that timing should be set to 6-8 degrees with wire unplugged.

I will assume that you have firing order on the plug wires correct (??).

Have you checked fuel pressure and fuel filter?

Checked voltage at the battery? Low battery voltage can cause strange things to happen.

Let us know what you find.

 

Any possibility the timing chain jumped a tooth?

 

Roller or flat tappet cam ? I hear a lot of stories about new flat tappet cams wearing out real fast. They say its the oil. I question the quality of the cams though.

 

That was the first thing that came to mind but I assumed that it was a roller block. I don't know enough to question the cam quality issue but I do know that break-in without the additive(mainly the zinc) on hydraulic flats with today's oils (thanks to the EPA I guess) and the problem is more than likely cam related.
Wiped lobes will cause what the OP is seeing right now

 

pull both rocker covers, loosen all rocker arm nuts till the rockers can be turned off from the pushrods, pull all spark plugs, rotate engine and check all 16 cam lobes with a 1" range dial indicator at (now) exposed upper ends of pushrods (keep dial indic parallel with pushrods to avoid "parallax error")

 

Ok, so far, we've checked compression on all 8 cylinders. 150 is the lowest of all 8. We have adjusted the valves, some were loose, but after adjusting the valves, it still runs like crap. Now the vaccuum gauge is steady at 15 inches of vaccuum.

We've pulled the timing cover and the gears/marks are correct. We made sure the main computer ground at the thermostat housing was good. We are not getting any codes. We blocked off the EGR with a plate and that didn't help. We've changed the computer/prom, timing module, knock sensor, distributor, map sensor and coil.

To answer a few previous questions, it's a roller motor and the sticker under the hood says set the timing at 0.

Oh, yeah, with a scan tool we can see that when the computer is in open loop mode, it runs ok, still not as good as it should, but a lot better than it does when it goes into closed loop mode.

 

A buggered up EGR value can cause problems. I bent one on a 94 C1500 with TBI. It would barely idle and ran like crap going down the road. I also spend a lot money and time chasing gremlins on my 96 vette only to find out it was a stuck EGR valve.

 

We currently have the EGR blocked off with a plate.

 

My parents had the same exact problems your having in their 95 tbi truck, after 4 bad computers (in a row) from Advanced Auto, they got the fifth one from GM(paid a little more) and it fixed everything, you don't say where you bought your computer from but if its a reman, I would replace it, no one expects to buy bad ECM's.

 

I used to be a Blazer 4WD enthuasist...... went through a lot of things with them, including your problem.

With me, the problem was the charcoal canister on one occasion..... see the below link which will explain a lot to you. (I had to learn it the hard way )

Not sure if this is your problem, but can definitely cause the symptoms you have described.

http://www.autozone.com/az/cds/en_us...rInfoPages.htm

Good Luck !

 

Since you said the problem occurs in Closed Loop, my first thought is to suspect the 02 sensor. Since you have a ScanTool, what readings are you getting?

Is the 02 voltage quickly cycling from about 100 to 900 mv? If it's stuck at a certain voltage or really lazy, that's what I'd attack next.

I once had a ECM that wouldn't take over the timing chores once the engine fired and the timing remained fixed at the 6 degrees I dialed in. Swapping my ECM with a friend's ECM (which was known to be good) pin-pointed the source.

If you know someone that has the same ECM as you do, see if you can borrow it for testing purposes.

Jake

 

We have monitored the oxygen sensor and it's switching like it should be.

We have a friend with a TBI Blazer and have already tried using his ECM/prom, but we still get the exact same problem.

 

check the cts (coolant temperature sensor)

 

This sticker under the hood asks for 0 degrees of base timing. You achieve this by unplugging the "set timing connector", set it to 0, then plug the connector back in and the computer does it's thing with timing.

We noticed that if we bump the timing up to 20 to 30 degrees of BASE timing, it runs a lot better (the computer add's roughly 20 more when the "set timing connector" is plugged in). I would say it's running at about 80%, but it's still not at the 100% it should be for a new motor and everything listed in my first post.

So giving it more timing makes it run better. We talked to JET chips and they claim that it's not their chip, since a stock computer/chip has the same issues.

Suggestions?

 

112233 by chance did your engine builder put on the wrong harmonic damper? When I freshened up the 85 I found a cracked damper. Got a new one which gave me the same condition you are having. Mechanic buddy used a piston stop to locate TDC. The damper was way off. We marked the correct TDC and made all timing adjustments from there. That was three years ago, still runs great.

I am not sure if I got the wrong damper or if the one I got was keyed wrong. Might be something to check.

 

We though of that too, we did the piston stop and when #1 is at TDC, the damper is right on.

 

Two things:

First, don't forget to disconnect the battery for about 30 seconds after setting the base timing with the connector disconnected. I know on the 86, the ECM will set a code once it recognizes that the engine was run with the connector disconnected.

Second, when I bought my 86 it was doing the same thing. I finally tracked it down to the ECM that wasn't taking over the timing chores once the connector was re-connected. The engine was only running with the base timing and what little was in the PROM.

I discovered it by borrowing another ECM from a friend's 86 and when I plugged his in, the engine perked up amazingly. I bought a remanufactured ECM from O'Reilly's for $100 and my problem was solved. You might want to try that too.

Hope this helps.

Jake

 

well, if it back fires through the throttle body, an intake valve is hanging open. if it back fires out the tailpipe, it is late timing.
If it runs good in open loop, it is obviousyou have a bunch of options.
1. it's a sensor
2. cracked dist. cap
3. bad plug wires
4. if you have an exhaust leak, the exhaust percussion will make the computer hear a "knock" through the knock sensor. remember the knock sensor is basically a computer "microphone", and an exhaust leak at the exhaust manifold will coincide with a cyl. pinging.
5. There is a possibility that a new timing cover could be the culprit.
If you bought a new timing cover, remember that about 1970, Chevrolet moved the number one reference point on the cover as well as the balancer.
Check both.
to do that, you need to check the piston position with the spark plug removed, and the timing mark on the crank lines up, the #1 rockers are loose (compression) and where the distributor rotor is pointing, the #1 plug wire on the distributor cap
6. The engine could be warming up and the valves could be hanging open if they only cold adjusted the valves, being too tight.
7. The distributor gear is made of the wrong metal, and is prematurely wearing out.
8. The cam was improperly broken in if it is a flat tappet cam
9. The intake manifold gasket is leaking, causing an extremely lean condition
10. cracked intake manifold.
11. other than that, I ain't got nothin

 

[QUOTE=vetteguy112233;1569163536 Oh, yeah, with a scan tool we can see that when the computer is in open loop mode, it runs ok, still not as good as it should, but a lot better than it does when it goes into closed loop mode.[/QUOTE]

When the engine runs better in Open Loop than in Closed Loop, the firt thing I'd check is the 02 sensor.

The 02 voltage should quickly cycle between .100 and .900. If it isn't, replace the 02 sensor.

Did you change the complete ECM or just the PROM? To solve my 86 timing problem I borrowed the entire ECM from a friend's 86. His made the engine come alive, so I knew it was my ECM, not just the PROM.

Try attaching you timing light to #1 then fire and rev the engine. You should be able to see the timing advance as you rev the engine. Painting the scribed mark on the damper will help you see. Nail polish worked for me.

Jake