Engine Performance

[Yankee7985]

This is my first post and I need some help.

 

I have a 95 Z71 with a 350, and it runs like crap and is only getting about 10MPG. Along with this, it stumbles during acceleration and backfires through the carb. It also has 170,000 miles on it.

As a result of this problem, I have replaced the EGR Valve, TPS, O2 Sensor, fuel pump, catalytic converter, MAP sensor, plugs, wires, cap, rotor, and EGR manifold.

I have run out of ideas. Any help in solving this would be greatly appreciated.

[Mule]

Here is something I had on one of mine once. It is just a guess, but the symptoms were the same. One of my rocker arms was cracked enough that it was hanging loosely. It caused backfiring through the carb, since it was an exhaust valve, and the exhaust had nowhere to go. I know this will involve removing the valve covers to check, so there is some work involved. But if you go in there, check the valve springs, the rocker arms and look for any debris that might be impairing smooth operation.

 

Also check fuel filter. Had that problem on a Toyata truck when the power dropped off and it stumbled a lot. Check this first - it's cheaper.

 

Air cleaner and intake path. Are they clean and unobstructed?

 

TBI unit - may be plugged up. When you say it backfires through the carb, does it have a real carburator or is it a throttle body injection (standard on 1995).

 

Catalytic converter. They don't last forever, and it may be plugged up. Or somewhere else in the exhaust may be plugged up.

 

You could have the dealer check for any codes.

 

Good luck finding the problem. Hopefully it is easy and not too pricey.

[Wingnut]

I thought 95s had the Vortec?....Guess not.

 

You listed just about all the sensors, what about the (anti)knock sensor, did you do that one? I don't think it would cause an actual backfire, but it sure runs like a$$ when it goes bad.

 

I agree that the cat could be bad, or the TBI needs cleaned.

 

Is is throwing an SES light?

[Mule]

Wingnut,

1995 are non-Vortec, OBD I engines. But they started using the center nut valve covers and a lot of people think they are Vortec. Even one of my mechanics looked at it and said, "It's a Vortec, you just have that Banks air cleaner on it." I have even had GM dealers tell me that the 1995 is a Vortec.

 

Oh, now that I have said that, I wonder if the ROM chip or the computer itself (behind the glovebox) have gone bad.

[Wingnut]

Thanks

[Yankee7985]

Sorry about the semi-confusing info. on my first post. The engine in this Z71 is a 5.7L, Vortec V8, with the TBI fuel system. In addition to catalytic converter and the other items listed previously, I did replace the fuel filter. As for the SES light, it is not coming on. This 95 does not have a knock sensor.

The throttle body is working fine, and there are no obstructiions to the air intake system

[dieselman]

 

If you are getting a backfire out the Tbody check to see if you have all the wires on the right plugs fireing order is 18436572 if ok do a quick compression test this will tell if you have a bad ex. valve or spring or rocker or bent push rod. You can also check the cap for a crack. If the truck is running bad at idle pull the plug wires off one at a time [use insulated wire puller] this will show you which cylinder is not working then do a compression test take the rocker cover off and see if it is a broken spring-loose rocker-bent push rod. If all is ok and there is no compression you have a burt valve and the head must come off. If you have over 150 psi compression then it is a plug or wire or cap. good luck

[Mule]

Now, I am confused.

I know my 1995 Yukon is not a Vortec, and I "thought" 1996 introduced Vortec with OBD II. Did the Z71 introduce the Vortec earlier? Maybe a late year model build - still a 1995, with 1996 parts? Is the computer under the hood or behind the glovebox? Is there a round air cleaner on top of the engine or a tube that goes off to the side with the air filter? Does the black plastic part on top of the TBI say Vortec?

 

I ask, since fixing one can be a lot different than fixing the other. And to increase my knowledge. Certainly not wanting to argue.

 

This is what I based my comments on (www.snoops.com):

 

Before 1987 all trucks came with carbureted motors. Introduced in 1987, TBI motors were introduced. TBI- throttle body injection, was a form of fuel injection. At a first glance it appears to be a carb, but it is in fact a computerized injection system with a set of injectors located inside the throttle body.

 

The year 1996, would be a breakthrough year for GM. Technology and innovation led the way for better performance under the name VORTEC. Vortec was a term used to describe the swirling effect (vortex) from the new head design. It didn’t stop there, Roller cam, increased compression ratio, and sequential fuel injection were some of the aspects introduced in the new engines. The jump from 1995 to 1996 was so great that they gained as much as 55 horsepower from the previous year in the 5.7liter.

 

Overview

 

In the 1988-1998 trucks, the TBI motor was used from 1988-1995.

Characteristics: Tons of power and torque off the line, but the power flattens out in the higher RPMs. Not a very easily modified high performance motor due to its design and TBI system. Gas mileage is not great on these motors.

 

1996-1998, the vortec 350 was used. Made 255hp and 330ft/lbs of torque.

Characteristics: Better fuel efficiency and more power throughout the rpm range. Responds well to bolt-ons.

 

1999-to present, the vortec/ls1 type motors were introduced. The 5.3l motor makes 285hp and 325ft/lbs of torque.

Characteristics: Tremendous power in the top end range, which makes it great for drag racing. Like the earlier vortec motor it responds great to bolt-ons and has great power band

[Yankee7985]

The engine is the 5.7L with TBI.

Just got done cecking the compression and all cylinders check @140psi. The computer is behind the glove box and it is not a OBDII system. All rocker arms, push rods, and springs also look OK. The vehicle was running lousy with the old cap and rotor as well as the new cap and rotor.

[Mule]

Yankee,

When you changed the plug wires, you did the one to the coil, too? I don't want to suggest throwing money at parts, but maybe the coil itself or distributor. Double check that the plug wires are in the right order.

 

You seem to have hit everything I can think of. Fuel and spark are the basics, so what's left?

 

Did it degrade over time, or all at once?

[Wingnut]

Flash the `puter maybe?

[Mule]

I didn't notice in the first post that you did replace the catalytic converter. Good compression hopefully means it is something simple, just not obvious right now.

 

Dang...what is left? Something electrical or electronic?

[Yankee7985]

Mule,

 

I replaced the coil wire as well. Also checked the coil I/A/W the given specs with a DMM. As far as throwing money at it...... been there done that to an extent. Luckily all the electronics were reasonable. The catalytic converter was $200.00.

I know it has an ignition module of sorts under the distributor.

The problems just seemed to start, and get progressively worse. I am running out of options, and cannot really afford to put the truck in the shop for 2-3 days.

[Mule]

Yankee,

You sure are thorough. Can you let the dealer have it for a few hours and get a diagnosis? Sounds like whatever they say is something you can fix.

 

I am familar with the ignition module that used to be in the older HEI distributors (with the coil on top), but not the 95's. They seemed to go completely bad at once, and the engine wouldn't start.

[Yankee7985]

Thanks,

I usually like to try and solve things myself, without paying somebody $90.00 a hour to look at it. Heck, we charge customers a little less than that to work on their $50,000,000.00 Gulfstream jets.

Looks like I will have to let them look at it. They will probably find a bizillion things wrong with it.