'96 Silverado Misfires

[96Silverado5.7]

First time poster...recently bought 96 5.7L Silverado 120k miles. Suffice it to say the seller was not an honest man and/or I am too trusting...

 

Day after I bought it got 2 codes:

1)P1345 crank/cam relationship error.

2)P???? left pre-cat O2 sensor low voltage.

 

What I did:

1) I cleared the codes with the OBD reader. I've never seen an 02 sensor code again.

2) P1345 came back same day

3) Took truck to shop, they used scanner and found ignition set 17 degrees out (should be 0 +/-2 degrees)

4) Shop pulled the distributor, found it off by one tooth, re-installed. Gear looked "GREAT"

5) No more P1345 or ANY error codes of any sort (I've since put 1700 miles on the truck)

 

Problem:

Still with an occasional misfire. Most reproducible when coming over the top of a high hill, occasionally a hiccup while going up a hill. Driving around town, very rarely any misfire.

 

What I've done:

Put a tank full of 93 octane fuel--didn't really help

Put a pint of Seafoam in tank with ~8gallons gas in tank--wallet is lighter, no other change

Bought a Scanguage to monitor the engine a little better

 

What I can tell:

-The plugs are new, gapped well, clean.

-Wires are very new appearing

-Cap/rotor looked new per shop (seller said was recently replaced)

-***Scangauge shows IGN of "-2.2" at 1200RPM, goes to as much as "-3.3" under load. I don't know if these numbers are advance or retard (and accordingly what + and - mean before the numbers). Amongst other things, I really don't have a grasp on this Vortec ignition system...

-If I keep the LOD (load in scangauge) below 25%, it never misfires.

 

Questions/Comments:

1) I suspect that the distributor is still out of spec, but I don't know what the Scangauge is telling me.

2) In researching this type of problem can be caused by any number of things (bad plug, wire, cap, rotor, cam sensor, crank sensor, vacuum leak, intake gasket leak, bad O2 sensor, bad ground, bad TPS, bad cat, carbon buildup wherever....)

3) I'm trying to approach things methodically rather than replacing everything that will unbolt

4) Again, most reproducible misfire occurs at the top of the hill, when coming out of the throttle (worse with cruise control on)

 

Any ideas? Thanks in advance.

[bill190]

That's right, all sorts of things can cause this! Try here...

http://www.random-misfire.com

 

Then oxygen sensors should be replaced as a regular maintenance item every 60,000 miles. This will also give you better gas mileage.

 

And so far as saving money (no guess and replace), best to get a GM factory service manual set for your year/model vehicle. These are 3 or 4 book sets. Then you can test various things. If they are per factory specs, then no need to replace!

 

Also these books have driveability symptoms, a list of things to check for each symptom, how to test, and what the test results should be.

 

For testing you would mostly use a multimeter and a fuel pressure gauge - sometimes an exhaust pressure gauge.

 

Here is on testing your fuel system...

http://www.asashop.org/autoinc/june98/techtotech.htm

[96Silverado5.7]

Thanks for the links.

 

I replaced the cap, rotor, wires, spark plugs. A little hesitation accelerating from idle was present before, now gone. Still having the hiccups on the highway.

 

I can't get over that the ignition is so 'retarded.' Starts -10 or so, gets worse with increased load. I don't think that the engine is all that rough, meaning so as to engage the knock sensor etc. My daily driver is an old diesel Mercedes so maybe I'm not in tune myself.

 

I am wondering if the the base timing wasn't set up correctly the first time.

 

Also considering pulling the plug on the knock sensor for a few minutes to see if ignition numbers improve on Scangauge.

 

Also will check the fuel trims tomorrow as I've read you can use these to get a sense if there is a vacuum leak.

 

Thanks for the help.

[bill190]

Basically there are electrical sensors which input information to the engine computer. Sensors which would monitor the following things would cause the engine computer to change the timing based on their input...

 

Crankshaft position

Engine speed (RPM)

Engine load (manifold pressure or vacuum)

Atmospheric (barometric) pressure

Engine coolant temperature

 

If one of the sensors monitoring the above was reading wrong or the wiring to one of these sensors had an open, short, or poor connection, then this could cause the timing to not be as it should.

 

I would check those before checking the base timing.

[96Silverado5.7]

This morning I tried to assess the knock retard with Scangauge, apparently this function isn't possible for my truck.

 

Maybe on to something else here....

 

I programmed the scangauge to monitor the short and long term fuel trims for Bank 1 and Bank 2. Values are as follows:

 

SFT1: -97

LFT1: 0

SFT2: -2

LFT2L 0

 

Seems like bank 1 is really getting the fuel dumped in. These values don't seem to suggest a vacuum leak.

 

Will keep looking along this fuel trim path for now.

[96Silverado5.7]

I interpret from the fuel trim data that, at least, Bank 1 is getting extra fuel somehow.

 

Recall that I mentioned getting an error code for the right pre-cat O2 sensor a couple weeks ago. I cleared it and it never came back. The O2 sensors are probably original all around.

 

So today I've now replaced the Bank 1 (left) pre-cat O2 sensor. Still getting the same driving problems. Fuel trim still shows that the computer is trying it's hardest to lean Bank 1.

 

I don't know if this fuel issue in Bank 1 is causing all my problems, but it's where I will continue to concentrate.

 

If the problem is limited to Bank 1, I assume that this would exclude a lot of issues (EGR, TPS, MAS, coollant temp) that should be causing mixture problems in both Banks. At any rate, seems like I should start to think about an injector problem. I assume this is something a shop needs to look at? I could have them check base timing same time.

 

Thanks again for the help/interest in my problem.

[superjim]

I think you are on the right track.

I would be checking injectors next.

And the timing. That does not seem quite right.

There should be more advance than what you are getting.

[Mike G]

Didn't the 96-98 have problems more related to the fuel regulator as well? Sorry, I am still working with the simpler TBI set-up on my old truck...

--Mike

[Boreal46]

With my ScanGage attached, I routinely show -20 to -30 on my timing while driving and the truck runs fine. (96 5.7L). I was told on one of the truck forums not to pay much attention to the readout due to the electronic timing. But I would think the technicians would know this as well, so I may have been given bad info. But I don't get any knock or dieseling, and I get normal (15-17 mpg) fuel economy. It does prefer slightly higher octane for better economy, but I don't get any pinging on 87 octane.

 

I am wondering if the the base timing wasn't set up correctly the first time.