Not Firing On All Cylinders

GMhunter93 · October 20, 2010

Ok so I recently brought my truck (1995 GMC sierra C2500 5.7l v8) a mechanic friend. He told me that He believed it was only firing on 4 of its potential 8 cylinders. It has run poorly for a while. I just put on a new distributor cap and rotor and I checked all the plugs and they all fired. It still runs very poorly any other things that I can try to get this thing working correctly? the plugs and wires are only 6 months old.

00Silverado4x4 · October 20, 2010

I have an old '76 Chevy pickup that when I bought it did pretty much did the same thing. As I was checking everything over & replacing my distributor cap I found my problem. I had a couple of plug wires turned around & 1 that refused to stay connected so I was only hitting on 5 plugs correctly. Also I've found that the underside of the cap on mine frequently has moisture build up so I'll spray it down with WD-40 from time to time, including inside both end connections of the plugs wires. After I did that my truck ran great. Sorry if this doesn't help. Good luck!

rodram · October 20, 2010

It sounds like you have full spark on all 8 cylinders so .. other possible issues to check include:

- fuel pressure... on this TBI (throttle body injection) engine the fuel pressure should be about 10 to 15 psi and there is not much forgiveness on the lower end... ie., 9 psi might start to cause performance issues like you describe ... if it is low then you likely need a fuel pump

- injector performance.. you will have two injectors spraying into the throttle body .. are either of them sputtering or are they both providing a good steady spray...? do the injectors need cleaning or replacing..?

- timing ... I don't know the intricacies of timing a TBI engine but someone should be able to provide this information

- coolant temperature sensor (the one right beside the thermostat) that tells your ECM to adjust the fuel mixture depending on the engine temperature.. when this fails the engine will run OK when cold but it will run rich when the engine warms up

Hope this helps ..

GMhunter93 · October 20, 2010

It sounds like you have full spark on all 8 cylinders so .. other possible issues to check include:

- fuel pressure... on this TBI (throttle body injection) engine the fuel pressure should be about 10 to 15 psi and there is not much forgiveness on the lower end... ie., 9 psi might start to cause performance issues like you describe ... if it is low then you likely need a fuel pump

- injector performance.. you will have two injectors spraying into the throttle body .. are either of them sputtering or are they both providing a good steady spray...? do the injectors need cleaning or replacing..?

- timing ... I don't know the intricacies of timing a TBI engine but someone should be able to provide this information

- coolant temperature sensor (the one right beside the thermostat) that tells your ECM to adjust the fuel mixture depending on the engine temperature.. when this fails the engine will run OK when cold but it will run rich when the engine warms up

Hope this helps ..

There could be something to this. When I start it up in the morning especially when it hasnt gotten warm yet it seams to run significantly stronger.

GMhunter93 · October 20, 2010

Also this ruck is not fuel injected, when the engine was rebuilt it got a carb put back on it.

rodram · October 20, 2010

If you now have carburetors on this engine.... then I have no idea what is going on... your coolant temperature sensor will not have any impact on the replacement carburetor system ... it only controls the electronic fuel injection...

from a fundamental perspective.. your engine might be starving for fuel, starving for air, or the timing may be way out ....

txab · October 20, 2010

Your engine came with a throttle body on it. It looks like a carb but it has two electric fuel injectors mounted on it. Do you still have this type of throttle body/carb? If so then you have a fuel filter under the vehicle that needs to be checked. Also if you have an electric fuel pump that needs to be checked for proper pressure and performance. Also you may have an O2 sensor that needs to be looked at.

But, if you have no electronic control of engine controls and its all "manual" then you still need to look into the fuel supply and spark control.

Supply more info about your engine. Saw in one of your posts that your 16, don't know how much mechanical experience/knowledge you have, but don't be afraid to ask questions. Give us as much info about your vehicle as possible

GMhunter93 · October 20, 2010

Ok I'm not exactly sure how to tell if it is fuel injected or a carburetor, how do I tell?

Kirkathon25 · October 20, 2010

check the crankshaft positioning sensor

AThomas7782 · October 20, 2010

Ok I'm not exactly sure how to tell if it is fuel injected or a carburetor, how do I tell?

TBI

Carb

GMhunter93 · October 21, 2010

I think it actually may be a TBI by the looks of those pictures. Where can I find the Coolant Temperature sensor? That sounds like a very possible problem.

txab · October 21, 2010

Many times it's in the cylinder head on the passenger side, might be on driver's side. Some vehicles had them in the intake near the thermostat. If you still have a TBI then you have an O2 sensor. It will need to be checked and maybe replaced.

GMhunter93 · October 21, 2010

So it is probly mounted over the passenger side cylinder head?

txab · October 21, 2010

Some vehicles had two temperature sensors/senders. One for the guage, one for the computer. If it does have two, usually the one near the thermostat housing is the computer sensor. If there is one in the head it will be on the exhaust manifold side of the head. Part might look like this:

Sensor

Sender

I'm not that familiar with the mid to late 90's 5.7 and their layout. Trying to go off memory