1997 C1500 with 5.0 vortec backfiring and bucking

gemson5 · July 24, 2015

I have a 97 C-1500, 5.0 vortec. I just replaced the spider injector last weekend. Had Cspi and went to multiport. I made sure all the injectors were seated and did not pull out when tugged on. Just put new plugs in and fuel filter. When I am driving with the engine around 1200 RPM or so. I am getting backfiring when basically coasting. Also when traveling highway speeds (65 to 70 MPH), I can feel sort of a bucking sensation. I had the truck diagnosed and the fuel pump is only putting out about 45 to 58 pounds. Would this cause the truck to carry-on like that? I am running out of ideas and money. I am going to check vacuum lined to make sure thay are OK. Any other suggestions?

oldscsc · July 24, 2015

Generally, these pickups are very sensitive to fuel pressure outside the spec range.

Normally the problem will manifest itself in the form of hard or no starts, but lack of power and/or multi-cylinder misfire are also symptoms of low fuel pressure.

If you have confirmed the pickup is making insufficient fuel pressure, the pump needs to be changed.

FYI, the 99-06 pickups' fuel pressure spec is 55-62 PSI. Your spec may or not be the same.

gemson5 · July 28, 2015

I am thinking of replacing three parts, ignition control module, throttle position sensor, and fuel pump. I looked on Mitchell on-demand through NAPA prolink an found the pressure should be between 56-62 psi. I am leaning more towards the fuel pump before anything else. The fuel pump is running about 45 to 48 pounds of pressure. I don't want to do the pump since I have to drop the tank and it might prove to be hard with a cast on my ankle. I need to have it running much better for our camping trip from Lancaster, PA to the Allegheny Mountains next weekend. I wish I had the money to just take it to a garage and let them fix it. Someone also suggested looking at the distributor and see if there is any play in it. I repalced the original plastic bodied one with an aluminum bodied on about 8 years ago. I don't think it would be the distributor. Any other suggestions from you guys ir gals? I am tired of the rolling far**. Ha ha. Hope someone has a suggestion or two.

00Silverado4x4 · July 28, 2015

It sounds like you have or going to check everything I could think of so the only other suggestion that I have is to double check the timing. It may have gotten bumped around and gotten out of time.

gemson5 · July 29, 2015

Just replaced the wires, TPS, and ignition control module. Still running rough and bucking and backfiring between 1000 and 2000 RPM. Not sure where to turn now. I was hoping this would cure the problem. Do I still change the fuel pump. Distributor is tight with not lateral movement. I am running out of hair to pull out. Help!!!!

grampadirt · July 30, 2015

Try cleaning the MAF sensor,might help.But be careful,those tiny wires are very sensitive.Use MAF sensor cleaner.

oldscsc · July 31, 2015

I would be glad your pickup is running at all considering its fuel pressure is 10PSI below spec.

It may not be a fun job, but I think the fuel pump needs to be replaced.

gemson5 · August 1, 2015

I just tested the fuel pump myslef and found it to be at 60 when the key is turned on and then runs a prety constant 50 while running. I am thinking the MAP sensor is eithe rbad or has a leak around it. The seal that slides on it does not look very good. Does anyone know if a bad MAP sensor would cause my running issue? Also the tining advance at idle read between 16.5 and 18.5 degrees. Where does the tining need to be as viewed on a scan tool? Sorry guys, but I am very frustrated with my truck. Heop I can get it runnung right this weekend.

1997SierraSLT · August 1, 2015

I have changed out the MAP sensor even though the ECM didnt throw a code and it did improve driveability. Im curious as to how you set the position of the distributor as it requires use of special scan tool

gemson5 · August 2, 2015

Got 97 running much better without backfiring and bucking. Set timing to where it should be. Here's how I did it.

Removed the # 1 spark plug. Took cap off dist. Tapped the starter to get the rotor pointing to # 1 tower. Turned the crank with my finger on the hole. When I felt the pressure pushing my finger off the hole, I knew I was close to top dead center. I used a 90 degree probe and inserted into the hole and felt the piston then had my helper, my son, turn the crank until I felt it start to go back down and then backed it up til it was at the uppermost part of the stroke. Loosened the distributor and turned the distributor until the rotor was pointing towards the mark on the distributor. CLamped the dist. down and assembled what I took apart. Took it for a test drive and was amazed it was that far out. I was going over 80 on the highway here with no bucking and backfiring. I even went through town and it ran close to perfect. It does have 171,000 miles on it. I am pleased it runs much better. Thanks to all who offered suggestions.

1997SierraSLT · August 2, 2015

glad you got it to run much better. In a pinch, that is how I'd do it as well to 'set' the timing.

If you ever take the MAP sensor out, be extremely careful prying it out of the plastic upper manifold. I got a little too aggressive once and it snapped off which resulted in me having to remove the entire upper manifold to fish out the broken piece of plastic.

If I recall, ECM will allow for a +4 to -4 electrical variance between the distributor rotor position and crankshaft position sensor. Thus, your distributor position can be off a few degrees and the ECM will compensate.

The ECM will throw a code if the variance is too far out of spec.

gemson5 · August 2, 2015

My MAP sensor was way too easy to pull out. The grommet sealing it is not in good shape. I am ordering one from a Chevy dealer close to me. Should be before my trip. In the mean time I RTV'd it so it won't leak. I love my truck and was so frustrated with it running the way it did. I replaced parts that probably didn't need replacing/ Like the ingnition control module was still original and TPS. I won't have problems with them as I put in Delphi brand of them. SO glad to have "my truck" back!!!!!!!