Hello Guys,

Longtime lurker, first time poster. 1998 Blazer ZR2 has a pinging/loss of power from 1800-2300RPM that has been driving me nuts. It developed this issue last fall and I've been chasing it ever since. No matter what I do, it still pings. Actually, seems to do it just as bad when the engine is partially warmed as when it's hot. I've done the following:

New intake manifold gaskets
Converted to new MFI spider assembly
Cleaned MAF, new air filter
New fuel filter
New dist cap, rotor, plugs (cyl #6 showed lean signs, but likely from old CSFI injector assembly)
Gibson cat-back (been on there ~ 10k miles)
Cooling system in great shape, can't hardly get it hotter than 195-200 even when sitting a bunch and 90F outside

On the scan tool, everything looks normal - EGR cycles appropriately, O2 sensors do not look lazy, TPS, IAT, ECT, MAP, etc., all look good.

Total spark advance when driving/under load ranges up to 34 degrees, cam retard is 0 to -2 degrees, sounds about right???

Idle is super smooth for a 4.3, right at 500RPM or so in gear - checked all around the engine with propane for vacuum leaks and didn't detect any, all looks great.

It DOES NOT ping at idle, revving engine in park, cruising on flat roads, etc. Only under load at 1800-2300RPM or so. Does it even on 93 octane fuel.

Which brings me to the latest check - my KOEO fuel pressure is exactly 60psi, which is the bottom of the range. When the engine is started/idled the pressure drops to 50-52psi. The system holds pressure well after shutoff for at least an hour or so.

At times, it's also a bit difficult to start hot (NOT cold, cold starts are excellent) only after it "soaks" for > 30 minutes. Immediate hot starts (at a gas station or something) are great.

So, do you guys think it may for sure be the fuel pump (maybe causing it to lean out), or does the new regulator with the updated MFI spider assembly keep pressure a little lower. I know the factory service manual I have says the pressure should not be below 55psi at idle but that's for the original poppet spider.

It's pinging for sure (not mechanical engine noise or something else) and drives me nuts - slight loss of power too. Let me know what you guys think. Also ran 2 bottles of techron through it at 3000RPM pulling a boat trailer in 3rd for about 2 hours. She's not carboned up.

Only 20K miles on the truck - long story.

 

TTT - fuel pump? Knock sensor?

Should I break down and take it to a dealer for a scan (hate to do that - got by all my life without it)?

Thanks.

 

There is a TSB (#03-06-01-024C) on a rattle noise in this RPM region that can be caused by torsional vibration of the balance shaft. The TSB details the replacement of the tensioner with a revised unit. This rattle could be picked up by the knock sensor and cause the PCM to pull timing, decreasing performance.

What are your knock sensor counts? How does the timing look through that RPM region?

 

You could try running a tank or two of 89 octane and see what happens. Could be the knock sensor, though I would look into what Kyle posted above.

 

He already tried 93 and it still didn't help.

 

My scan tool (Autoxray) isn't sophisticated to look at knock sensor counts but it does not appear that the PCM is pulling any advance out under acceleration. It almost always goes directly to roughly 34 degrees total advance under acceleration and I can't tell that there's any knock retard thrown in there when watching it.

My service manuals (factory/Helm) do not give any range or curves for a rough approximation of the correct spark advance vs. engine RPM - granted it's not hardly that simple, but do you guys have any idea what total advance these things typically try to shoot for in that RPM range within a normal (70F) outside temperature condition? Just curious if that's way out there in left field or about right.

Seems like my old IROC TPI was close to in that range, but it had higher compression. The owner's manual recommended 93 for it so I always ran premium and never had a ping, even after I bumped up the base spark advance a couple degrees.

Probably should have replaced that puppy (knock sensor) when I had the distributor and intake manifold off.

I saw that TSB awhile back on the timing chain and that's a good catch. It may yet be that - I don't hear it revving the engine in neutral or park though but maybe one wouldn't necessarily?

Should I bite the bullet and let the dealership put their real scan tool on it? May be worth $35 or whatever if they can find something I can't with mine.

Thanks guys, I really appreciate the help. I've been working on Chevys for years but this one has me stumped.