looking for help 1997 chevy 1500 v6

I have a 1997 Chevy 1500 with a v-6 vortec. I have had it to 3 shops trying to get a diagnosis. when the engine is cold, it runs rough / stumbles. this occurs at park when idled up or when in drive and applying throttle. not sure at what temp it corrects itself but soon after the temp gauge passes 180 or so, I can see by the temp gauge that the thermostat is opening and closing. I have had it to three shops and read P0300 code for intermittent engine misfire but no other codes and no dash light. I cannot find the problem. Help? I live in Southern Maryland and would be willing to drive an hour or two to find someone that can diagnose AND find the problem. Thanks for reading I am open to suggestions.

Do you have a scantool where you can provide STFT and LTFT values?

Nissan Technician

Is 1500 the Engine model, or the vehicle model? I'm not super familiar with Chevy, though I have worked on a few.

There are a LOT of different things that can cause this. You're going to need a scan tool that does live data to be able to read values from the engine computer.

If you don't have that, there is nothing else that you can do except old fashioned troubleshooting ( which basically means checking each and every part one by one ). It can work, but it takes a long time, a lot of work and also a lot of tools.

Without more information I can't really help you much, but here are some general things to look at / think about:


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Edit:
As you can probably see from the long ( and growing ) nature of the list of tests below, the value of a basic scan tool that reads live data is highly valuable.

Do you have to have one?
No. But it can tell you in 2 minutes what it might otherwise take you 2 days worth of manual testing to figure out.
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• Most engines are designed to run rich when they are cold. Perhaps you have a malfunctioning temperature sensor reporting back false temp reading to the computer.

This can go wrong 2 different ways:

1. It tells the engine that it's warmed up when it is not. This causes the engine to NOT run rich when it is cold which will not run right.


2. It tells the engine that it's still cold when it is actually warmed up. This causes the engine to continue to run rich when it should have already switched out to normal fuel mixtures.

If given the symptoms that you stated, I think that #1 would apply to you more than B. But you just don't know until you check.

• Maybe you have a malfunctioning Intake Air Temp sensor. If it is falsely reporting high temperatures, it would cause the mixture to be incorrect when the engine is cold, but become more correct as the engine warms up.


• Maybe you have a worn out ignition system that can't handle burning the rich mixture when the engine is cold, but can burn it properly when the engine warms up and goes to normal fuel trims.


• Do a spark jump test. Ground a screw driver, un-plug one plug wire at a time while holding the screw driver in its place.
See if the spark can jump 1". The spark should be thin, blue and snappy.
Only test one plug wire at a time and reconnect it when you are done so that you don't mess up the order of your plug wires.


• Check your spark plug condition, learn how to "read the plugs" ( an easy Google search )
• Check your plug wires and connectors for spark leaks and resistance that is out of spec.
• You can check for spark leaks using a grounded screw driver. Ground it, then drag the screw driver all along the top, bottom and sides of all of your plug wires.
If you see or hear any sparks... you've found a leaking spark problem.


• Check your distributor cap and rotor button for wear ( I'm assuming you have one of those on this 97 ).
• Check the resistance of the primary and secondary HV coil.


• Maybe you have low compression due to engine wear that gets better as the engine warms up. Have you done a compression test?
• What does the engine sound like when cranking while cold ( with the ignition disabled )? Does it crank evenly, or do some cylinders crank faster than others?
If this is happening you'll hear it.


• Maybe you have too much play in your valve / rocker arm assembly ( not sure which type this engine uses ).
This could cause slightly off valve timing, but would get better as the engine warms up because the metals all expand and fill up some of the gap.


• What type of fuel are you burning?
Burning fuel with an octane rating higher than what your engine is rated for could cause this.
Does your fuel contain water, alcohol or too much methanol?

Here is a way that you can do a basic test:

1. Go to trust worthy gas station that sells non-ethonal gas of the correct octane rating for your engine. Buy 5 gallons of that.
2. Check it for water using this test:
While you are at it... check your own fuel for water too ( if you have the means to remove enough from the tank )

3. When your fuel is low ( prefferably empty ) add the new non-ethanol fuel and test drive it several miles.
If the problem goes away, you know that the fuel is part of the issue. It doesn't mean that it's 100% the problem... but it could tell you a lot.

There are MANY other things that could be checked, but those are the easy things off the top of my head that you can pretty easily check with minimal tools and no scan tool. You'll need some type of Volt/ohm meter to check those temp sensors and plug wire resistance measurements though.

If you are in a pinch and looking for a way to get scan tool data, you can get a USB scan tool called an ELM327 on Ebay for about $11. I have one. It works well for basic live data.

You can also go to most auto parts stores and get a scan done for free. Normally they just scan for codes and that's it... but if you either find a nice a guy or have a friend that works there you can probably let your engine cool down and get them to read the basic live data as it warms up to give you an idea of what's going on. Make sure you bring a camera. Use it to take snap shots of the scan tool read outs on the screen.

Last but not least, if you have an Automotive school in your area they will often fix/diagnose your car for free ( you pay for parts but not labor ). They do this because they benefit from having real test cases for students to learn from. You benefit because you can get a free diagnostic supervised by someone who probably has spent half their life or more in the field. It's win/win.

Hi this is Ruben, Hope this helps you. Im very familiar with this system and have fixed this challenge a handful of times.


Have someone put a Fuel pressure gauge on the rail supply line. GM always puts a port there to test and this is the only way because all of the fuel injectors sit inside the intake manifold. Most people will refer to it as a spider fuel injection system.

So once you have pressure gauge on the rail supply line, have someone lend you a scan tool and go to Injector balance test( It may be called something different on other scan tools. Anyway, while looking at gauge select injector 1 and fire it. What the scan tool will do is turn on the fuel pump to full pressure, so document the static pressure then fire the injector and see how much the pressure drops when the scan tool fires it.

you want to see consistent fuel pressure drop on all of them.
What I found there is usually 3 to 4 of the injectors that are dumping a ton more fuel then the rest.. What ive also found is that the connections at some of the injectors cracks because its all plastic high pressure fuel lines that crack and consistently leak fuel into the intake manifold and cause misfires.

I just fixed one of these last week on a chevy van with exact same symptoms and same set up.

when you fire the injector with the scan tool you will hear the injector clicking when it fires they are actually quite easy to hear.

Hope this helps but im sure this is the source