Shudder or miss at cruising speed
 

Thanks in advance for any help or suggestions....

I have a 99 Tahoe 4x4 5.7, 189,000 dependable miles. Within the last month I have had several problems surface. It started with #5 poppet injector hanging open and hydro-stating the cylinder. Not knowing what had happend and being stalled in the middle of a busy street, I broke the nose of the starter off by trying re-start it. Long story short...had it towed to what I believed was a reputable shop. The spider assembly was replaced with the newer MFI version (OEM), along with a new starter, and I was back in it after one day.

After the injector work, I noticed a misfire or "buck" after reaching cruising speed and going to light throttle. If I keep it under load...no problems. I returned to the shop because I didn't have this problem before the injector work. I told the guys something wasn't right. They scanned for codes, misfires, and found nothing. Zero codes or misfires. They claim that the engine is sound and that I now have transmission problems. Not what I wanted to hear but at 189k, I could'nt argue. Maybe my luck had run out. Took it to a reputable Houston area transmission shop, they said it felt like torque converter shudder, but recommended a complete overhaul. I accepted. Now it's worse. The tranny shop says that it's worse because the tranny is now tight and firm. I am starting to believe him. By the way, the drive line is tight, no slack or back lash.

I have found some similar threads to this issue, but no solutions.

Here is what has been done so far:

New MFI injector spider (1500.00 installed with new starter)
Trans over haul (1780.00 with new converter)
Standard tune up (plugs, cap, rotor, wires)
New OEM fuel pump

As I said, I am leaning towards a motor problem. I think I was mislead on the tranmission issue. There were no problems at all before the injector. Has anyone experienced this "bucking" at highway speeds while cruising. Really its at any speed above about 40mph.

Please help

double check the plug wires for any signs of misfire or cross firing make sure they are in the holders and not flopping around on the block

if it only does it when it is warm then I would try a new distributor and ring gear.

My 1998 is doing the same thing after the fuel injector change over. Was running fine but wanted better performance so I did the conversion and now have this problem. Mine is a very hard bucking while under load and lasts a fraction of a second

No Luck so far
 

Ok, no luck so far. After double checking the plug wires, everything seems to be good. Wires are new, connected correctly, in the wire holders, and not laying on the block or exhaust manifolds. There are no visual signs of arcing or burnt wires.

Lariders mention of the distributor was interesting to me. I had heard this was common. With no mis-fires or trouble codes I was unsure, but since the subject kept coming up; I thought it would be good to check into. I pulled the distibutor and sure enough, it had excessive wear on the gear. I replaced it with a remanufactured unit, but I am still having the bucking. Thanks Lariders, regardless I eliminated a future problem. This gear was in pretty bad shape. I have attached a picture. It doesnt show very well but...

Thanks for everyones input so far. If you can think of anything else to check please let me know. I don't think that I am at risk of walking here, it is just very irritating considering the amount of money I have invested so far.

what all did you replace this time? did you get a whole new distributor?

Yes. A new "remanufactured" distributor w/gear and cam sensor. Could a bad or fouled EGR cause these syptoms. I am hung on the fact that it wasn't doing this prior to the injector failure.

Banging my head on the wall
 

Ok folks, an update. I drove the truck from Houston to Dallas to have a family friend who owns a garge look at it. I thought at a minimum I could eliminate any lack of trust by talking to a family friend. He scanned for codes and found 1. The cam retard was off -16 degrees. Oddly, one of my attempts to solve the problem was to install an performance chip in the ecm, and it did not trigger an CIL. I removed the chip on the spot and a light immediately came on. Therefore no codes were present until I removed the chip. We re-set the cam retard to 0 degrees. But....the problem did not go away. He, like the folks here in Houston, say I have a transmission problem and referred me to a trans shop in Tyler. I drove over and they contested that it was not the transmission. Basically the same song I was hearing in Houston. Needless to say, I drove home slightly dissappointed. On the drive home, I got a CIL light. Checks showed P0420, converter low efficiency Bank 1. The same bank as my original injector failure. I had the converter replaced and the light has not came back as of yet.

This past weekend, I drove to San Antonio for a family vacation. My gas mileage has definately improved since adjusting the cam retard. I'm getting about 15.5mpg. I have burned premium gas for at least the past four tanks of fuel. In addition, I have replaced the plug wires with AC Delco wires, and checked the plugs for any signs of fouling. A couple of them had a lot of carbon build up, yet otherwise ok. I replaced the EGR valve thinking that it could be a contributor, but she still bucks at cruising speeds.

The truck will not do it when cold. When the engine reaches operating temps and goes to closed loop, what happens? What problems could've been created when the injector hung open? There were no problems until the injector spider was replaced. Should the ECM have been re-flashed?

Not sure how the cam got retarded 16* but has the fuel pressure been checked? Don't rule out a bad plug. I've seen plugs that were cracked during the installation process.

BTW all premium gas is doing is lightening your wallet unless the ECM has been reprogrammed.

Same Issue
 

Did you ever find out the problem? I have a 1998 Chevy Z71 pickup and I have the same issue. It started after the Spider injector change.

Thanks,