Bought a 1998 K1500 Silverado Z71 with about 167K on her and the first time I had it on the interstate it seemed fine, but the first long hill the SES light started flashing and the whole truck started shaking. P0305 Cylinder 5 misfire. Just thought it was a bad spark plug or something, so not knowing the vehicles history I changed out all the ignition stuff, new rotor button, distributor, wires, and plugs. Re-routed wires for separation. Still starts shuddering at throws P0305 at speeds over 55mph while pulling hills, worse shaking if towing. Mechanic told me it might be a sticking exhaust valve. We've checked valve lift and that was good, valves are very clean for this year, everything good. Checked compression in each cylinder, all 8 reading the same at around 180psi on a brand new gauge. Decided this couldn't be a damaged valve or ring. Pulled intake and found an intake leak, installed new gaskets and sealant to eliminated that as a potential issue. Still threw the P0305 code immediately on test drive. Checked grounds, cleaned, all good. No change. Checked fuel pressure and did a bleed down test, all good there. I had just put a new sending unit, fuel pump and filter on. Cleaned Mass air flow, new air filter, no change. Took a test drive while my mechanic friend watched the scanner and it was retarding the timing advance under acceleration instead of advancing it. Prompted us to replace the knock sensor. That was the first positive change in drivability. The misfire and shaking is less pronounced, it's not knocking drinks out of cupholders anymore, but still throws p0305 on hills. Decided it had to be a bad spider injector. Replaced that last night and it's the newer MFI style, much nicer, and the old one looked pretty fried on cylinder 5 when we removed it. Was confident we found the issue. Nope! Took it down the interstate and while the timing advance and all the numbers on the scanner appeared to be good, the first steep hill and she started to lose power and was popping out of overdrive a lot and then the dreaded P0305 again. I'm just at a loss here. Anyone know what's going on I'm so tired of throwing money and time into this. It's an incredible looking truck for the year I'd love to be able to enjoy it and be able to tow with it occasionally. Right now all it's good for is sub-45mph around town stuff.
1998 5.7L Vortec Persistent P0305 misfire
1 reading
justins1998
21 - 40 of 56 Posts
Joined
·
17 Posts
So what would the symptoms be for a bad distributor? We had it out and inspected it when we were replacing the intake gaskets. I didn't notice any wear but wouldn't really know what I was looking for.MMZ71 said:
Joined
·
440 Posts
Worn gear. Broke pin that holds the gear on the shaft. Bad bearings (hard to rotate the shaft). Slop in anything. Rusty crusties.
That's about it.
I would also try a new pick up coil and ignition module. Those things can make it fire funky if they are on their way out. And the way to replace those two items is a new distributor. If you go this route, keep your old distributor. I got one the other day that was bad out of the box and needed the old one to confirm the issue.
That's about it.
I would also try a new pick up coil and ignition module. Those things can make it fire funky if they are on their way out. And the way to replace those two items is a new distributor. If you go this route, keep your old distributor. I got one the other day that was bad out of the box and needed the old one to confirm the issue.
Joined
·
17 Posts
Any suggestions on the best quality brand to get? I've hear MSD and Accel are crap now.
Joined
·
3 Posts
I also have a 98 Z71 with the 5.7L engine, and last time I got it smogged it failed due to a misfire. It took my mechanic a week to diagnosis the real issue, along with a factory rep for the analysis equipment they use. Turned out to be worn out hydraulic valve lifters ( along with lower oil pressure due to the miles on my engine). Lifter was not pumping up to correct height. New lifters solved my problem.
It took some very sophisticated analysis equipment to find it, though. hope this helps, if you know someone who can do the diagnosis.
CarloPel
It took some very sophisticated analysis equipment to find it, though. hope this helps, if you know someone who can do the diagnosis.
CarloPel
Joined
·
440 Posts
'justins1998 said:
OE acdelco if you can even find one.
Edit... I went and looked at rock auto, they have a delphi and ac delco that are $234 and $261, but those should be the best available. I would absolutely be cool with the delphi that's a little cheaper. Delphi makes very high quality parts. OEM level. So does ac delco. However... we put a $80 one in the other day and it's doing just fine.... another however, we put a $80 one in another truck recently and it wasn't fine. So the cheapies are hit and miss. The MSD was a pig with lots of lipstick on it.
Joined
·
17 Posts
Okay so I put a new quality distributor in, new cam sensor included, and I put in a new ignition module and ignition coil. Also a new Mass airflow sensor. Greatly improved, I took it down the interstate and set the cruise at 75 and waited for the hills. Was doing better than ever before. It climbed several hills without issue, but at the very top at one of the longest hills it down shifted and threw the P0305 code again, although practically no shaking of the truck. I'm wondering if I'm getting closer, but there is no real timing adjustment on these vortec distributors. Actually, I had to use an old style clamp so that I could rotate the distributor a bit to get it within the -2 to 2 degree tolerance to get the crank/camshaft correlation p1345 code to go away. Thoughts?00pooterSS said:
Joined
·
440 Posts
Probably a stretched timing chain if you couldn't get the distributor to line up like it should using the keyed retainer. If you had to over rotate the distributor something that turns it isn't lined up. Whether it be the cam timing is out or the distributor gear is off or distributor was stabbed wrong. You may want to see if you can get the original distributor to line up on the keyed retainer and be able to hit the + or - 2
If it will and the new one wont. Something is going on there. If neither will then I would suspect the timing chain being stretched.
Still concerned about valve springs too.
If it will and the new one wont. Something is going on there. If neither will then I would suspect the timing chain being stretched.
Still concerned about valve springs too.
Joined
·
17 Posts
I could understand about the valves and I have a set to go in that has a 120 lb closing strength instead of 80 lb. We are just trying one thing at a time before we tear into the engine again. There was a little difference in the way the new one was clocked, either that or the old one was tweaked...We had to rotate the new one a hair it to get it to match. The guy I've been working with is a friend, he was certified up until about 2004. He said he checked a few things and didn't believe it was a timing chain issue.00pooterSS said:
Yesterday I got a hold of a little bit better scanner and used the OEM advanced function and got several powertrain codes that don't show up on the DTC scan...
Attachments
![20210622_215052[1].jpg](/d3/attachments/112/112484-d0db2ea86104258b47f411a712dd5c1c.jpg)
Joined
·
440 Posts
holy crap that's a lot of codes
The only real one i'd be concerned about at this time is possibly the control module codes and the insufficient engine temp code. Does it run cool all the time?
Should run right around 200
The o2 sensor heater codes and cat codes shouldn't affect anything
The only real one i'd be concerned about at this time is possibly the control module codes and the insufficient engine temp code. Does it run cool all the time?
Should run right around 200
The o2 sensor heater codes and cat codes shouldn't affect anything
Joined
·
440 Posts
You can turn the crank back and forth and check for chain slop too.
Here's an article on it that sums it up for me/so I don't have to type it out.
http://www.misterfixit.com/chanslop.htm
Here's an article on it that sums it up for me/so I don't have to type it out.
http://www.misterfixit.com/chanslop.htm
Joined
·
440 Posts
Joined
·
440 Posts
And there's a vid that should be helpful too.
Joined
·
17 Posts
I have a GM tech2 clone coming. Bound and determined to get to the bottom of it. I have another truck that needs the ABs bleed capability so I just went ahead and bought the scanner, and if it ends up being the PCM I can reflash the new one with it. However, looks like a wiring or ground issue could cause these 601-604 codes so we are going to run through those first.00pooterSS said:
Joined
·
440 Posts
Gotcha. Best of luck man, that's a lot to handle there.
Joined
·
17 Posts
Just to update after a while trying different things. The computer swap went well without a hitch. reprogramming successful but no change in performance the miss is still there and getting worse it seems...cruise control wont even stay engaged on the interstate now. New coil and ignition module seemed to make things even worse. I've already had to replace those with a third so it actually starts good. Want to drive it off a cliff.
Joined
·
440 Posts
did you ever try valve springs
Joined
·
17 Posts
I haven't been able to. I'll have to hire a mechanic I think my friend is tired of working on this truck with me. lol
I have a customer with the exact issue you are facing. He likes to do his own work and I told him to check for broken springs. His misfire code sets a P0303, but at 2500 RPM in neutral it shows misfires on #5.
I'll update you when he gives me feedback.
I'll update you when he gives me feedback.
Joined
·
440 Posts
They're fairly easy. A basic spring compressor from autozone will do the job. Valve covers are easy to pull.justins1998 said:
Just need to fill the cylinder with air or string to keep the valves from falling down while pulling the springs.
Customers valve springs were fine, but what fixed his is the simplest thing, and I don't know why I didn't think of it on his first visit. 
It may cure yours as well, specially since you replaced the ECM.
Crank relearn.
It may cure yours as well, specially since you replaced the ECM.
Crank relearn.
21 - 40 of 56 Posts
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
