I own a clean, well maintained 98 Z71 pickup as a daily driver. Im having trouble with it that I cannot figure out, and im having a hard time finding anyone that really knows these engines to help me out. I dont see an appropriate forum for this engine so im hoping someone in this section can help me out.

I bought this truck with 30k miles on it and it always ran really strong. 10,000 miles ago, the intake gaskets went on it (as they all do) and so i replaced them myself. The swap went flawlessly, until I drove it i discovered it had a high RPM vibration it never had before. It starts at 4,500 RPM and goes all the way to the 5,000 RPM shift point, and enough that i can hear and feel it through the vehicle. It is directly related to RPM. I cannot figure out what it is because the only moving part i tampered with during the intake gasket swap was the distributor and the radiator fan. The radiator fan is pretty darn simple, not sure what could go wrong there and nothing seems wrong with the distributor either. Is it possible to install the fan the wrong way?

edit: i did also replace the accessory belt with a new gatorback.

Today, I went over my buddies and we hooked up the truck to HPtuners and dialed the fueling and spark in perfect. The thing is a MONSTER out of the hole now, lights up both 285 BFG rear tires like its nothing. But in my data logs i noticed the truck was pulling timing north of 4500 RPM and discovered that it is doing that no matter what the timing is set at. In other words the knock sensors are picking up the vibration as false knock and im losing horsepower because of it.

Its been this way for 10,000 miles and has been fine but I've been asking all over and everyone is clueless. I don't know what is left to check, but I'd like to get it fixed.

I can get a datalog if it helps.

 

It could also be a vacuum leak. Check for DTC's and try data logging if that fails. It should at least point you in the right direction. Also make sure you harness plugs are all the way in. A loose connection can do all kinds of strange things.

 

Did you use the scanner when you put the distributor back in? It's possible to have the timing off and it still run. Check your tps for smooth operation mine gave me fits on my 98 until I started using acdelco parts

 

Those trucks are commonly known for injector poppet nozzles sticking. GM has even developed an updated injector assembly without the poppet nozzles as a retrofit kit. Do an injector balance test with a tech 2 to see if that is the problem.

Edit- I have also seen quite a few of them with leaky fuel pressure regulators. When they leak, they dump fuel into one cylinder. Read the plugs and see if one looks pig rich.

 

reading all this, we might be on to something here. The truck has been throwing a code for a bad oxygen sensor lately. The driver side bank did look like the activity was a bit sporadic, however ever since we dialed the truck in with HPTuners it has not thrown the code again.

I dont have a tech 2, is there anyway i can check for injector balance in HPTuners? Would removal/reinstallation of the injectors have caused this? Im not even certain i ever removed them from the intake. Is it possible i have an injector criss-crossed?

The timing was a bit off when i put it back together but it seemed to self-adjust for the most part. I never quite got my perfect idle back but its close. But like i said i dialed the timing back in with HPTuners across the board. There is still a very very slight shake at idle (prior to the problem it was difficult to tell the engine was running it idled so smoothly)

 

The distributor needs to be within +/-3 degrees. Not sure if you can get that data with HPtuners though its called cam retard. I think there is also a crank/cam sync relearn that you can do after getting the dist set properly. The dist affects injector timing too not just ignition timing.

 

after driving it today the truck is running way too good for me to think any timing is off. It is smooth as butter until i get into the high RPMs. I also checked the injectors, they all connect at one block so nothing is switched. I checked the spark plugs on the drivers bank too and they all look fine.

 

You should listen to the gentleman. Turning the distributor is how you set the injector timing and rotor fazing. I know a Tech2 will give you the cam (cam position sensor, actually) retard, like he said I'm also not sure if HPTuners scanner will. The shop manual gives you a procedure for installing the distributor that will get you very close-to-dead nuts. It has no effect on ignition timing, only the injector timing and rotor fazing. You should check it. I have owned three of those, tuned more than I can count.

Good luck, Ed

 

Did you ever experience any high RPM vibration with them?

When i did the intake i used some kind of shop manual to align the dist, but honestly forgot what it involved, its been almost a year. We borrowed it from a mechanic we know. I was aware of the +3/-3 limitation and so when i noticed the vibration i tried turning the distributor to several different locations. (i started from one extreme and progressed to the other) No matter what setting i tried, the engine stumbled very slightly at first and then compensated for it within a few seconds. The vibration in the high RPMs remained no matter what the setting was - and never changed. I tried this while the engine was running and i also tried shutting the engine off and resetting the computer between changes, same result either way.

After putting a few miles on the truck today it runs SO smoothly i cant even feel it running anymore at part throttle. the idle is close to perfect as well, and Ive seen a noticeable increase in a gas mileage. Everything is perfect until that 4500 mark.

Also... maybe i need something explained to me but i thought rotor phasing was everything to do with ignition timing? (aside from how the PCM modifies it)

I am going to look around in HPTuners and see if i can find anything to do with cam retard.

 

Under PIDs HP Tuners has these under engine diagnostics:

"Cam Signal Hi to Lo"
"Cam Signal Lo to Hi"

 

How about the tranny and motor mounts? Mine would "slip" when the tranny would rap up from the tranny mount having to much play it only lasted 211500 miles. J can only guess the slipping would be from the driveshaft sliding back and not fully engaging

 

I dont think thats it, the truck seems solid and ive only got 93k on it.

I just set up an appointment for wednesday with a contact at the local dealer to check the cam retard, he has a tech 2 and can do it for me for free.

 

Turning the distributor in a Vortec truck engine has nothing to do with spark advance, only injector timing. You wouldn't believe some of the phone calls I get, like "I hooked up my timing light, and when I turn my distributor and the timing mark doesn't move, what the hell did you do to my computer?"

The crank position sensor in the timing cover is used by the computer, not the distributor. The cam position sensor (for injector reference) and the rotor is all there is in the distributor.

 

ok thank you. well we should be able to get it spot on with the tech 2 and then go from there.

 

well i just went to the dealer. Appears i have a bad oxygen sensor (bank 2 sensor 2), the truck has thrown a code for that when it gets cold out, which it is right now.

However, the tech 2 scanner had nothing for checking cam retard or cam position. I even looked through it with the guy myself and they have the newest one. So for now I am going to replace that oxygen sensor but i have no idea where to go from there, i dont think its the oxygen sensor thats causing this.

 

The dealer just ripped you off for a new O2 sensor. Sensor 2 has nothing to do with running the engine.

 

they didnt rip me off, they just read the DTC code to me and told me i needed to replace it. Im going to fix it on my own.

I agree it is nothing to do with the problem im having, even if it is bad.

 

I understand. Probably no ill intentions on their part, just lack of knowledge, telling you that you "needed to replace it". You could solve the problem just as well by putting in a free discarded front O2 sensor. Front sensors should be replaced long before they totally fail and throw a code, in the interest of maintaining decent fuel mileage. I've never seen a rear one throw a code in any of my GM vehicles, up to 300k miles, so I take that as meaning it doesn't take a very good rear sensor to keep the OBD2 happy.

 

Im going to look through HPTuners and see if i can just disable the stupid things, i figure it has to be in there for people that dont run cats.

 

Now you're talking I didn't realize you had access to do that; so I just suggested the backyard method. I don't have HPTuners, but I'm pretty sure that's what pcmforless used when he tuned my '98 L31 pickup, and he disabled my rear O2's.