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My baby is starting to break down!

My baby is a '97 Z28 (LT1) with approx. 180,000 miles. Here’s the problem: under acceleration or load at >3000 rpms the engine feels like it is misfiring and/or backfiring. It runs roughly, only while giving it gas. It will still idle smoothly but struggles under load.



Now here is some background information of something that preceded this problem. I recently replaced the water pump and all of the radiator and heater hoses, refilled the system and bled the air out. While idling in a parking lot several weeks later, the engine got warmer than it had been since I’d replaced the hoses - I don’t usually have it idling for long periods of time so it had not had the chance to warm up to the point where the fans kick on. As it got hotter, the lower radiator hose blew off - I guess I didn’t have it tight enough… So I shut it down immediately, got the hose back on the best I could without getting it up in the air. I limped it to a garage where I could get it up on a lift. During this time, the engine got upwards of 250 degrees, because it wasn’t holding pressure, and the system wasn’t bled.



Once I got it up in the air, I got the clamp tightened up, filled and bled the system and everything seemed to be okay with it staying about 180 while I’m driving and getting to about the normal 220 when idling.



The day after this happened is when I noticed the car breaking down under load or acceleration after it was warm. I feel like this is likely an ignition issue, but want to get some other opinions before I start throwing money at it.



It’s been about 90k miles since the last set of plugs and wires, but until now, it pulled hard up to redline with no flat spots or hesitations. Everything else has been largely untouched.



It is not hard to start, it doesn’t leak or seem to consume any fluids (other than guzzle 93 octane) or smoke.



I’d like to get her back her normal reliable self. Suggestions?

Given 90K since plugs & wires I would start there. Also replace the fuel filter if it hasn’t been done in a while. And check/replace the air filter. Stop back & report then.

Check Engine Light on? Pulled the codes?

Ninety thousand miles is way too long on a set of plugs although this may not be the current problem.
You might consider the MAF sensor being at fault. Unplug it and see what happens.
Another possible cause is a clogged fuel filter and/or failing fuel pump.

I would also be a bit concerned about those temperatures. Even 220 sounds a bit much not to mention 250. Are the cooling fans cycling on?

I think your baby is fine. You have an ignition problem.

If the radiator problem resulted in some antifreeze being sprayed around the engine compartment that will impact your ignition system. If your ignition wiring has “moisture” on it that can cause some electrical arching and misfiring is the result. If there is some antifreeze residue on the motor do your best to clean it all off and it will help.

These are the exact symptoms I encounter in my '04 T’bird when one or more of the ignition coils goes bad. Since you have 90,000 miles on the plugs you’ve certainly gotten your money’s worth out of them. Replace the plugs. If you have COP’s (coils on plug) such as my T’bird if the new plugs don’t resolve it one of your coils could be going bad. If you have plug wires, distributer cap, rotor, and such replace all of that stuff.

With the antifreeze cleaned up and new ignition parts your baby should be very appreicative and willing run up to the red line anytime you ask her to.

Thanks for the quick replies!

Here are some answers.

Cigroller - I have the SLP dual shotgun (two 3" intake tubes) with dual conical filters. Cleaned and oiled at the same time the hoses and pump were replaced, since they had to come off anyway… Fuel filter, replaced recently, but the engine doesn’t starve. It pulls like a mule from 3000 to 6200 rpms with not flat spots or hesitation. Plugs & wires - Champion dual platinum, Taylor Spiro Pro 8mm silicone. Again, no problems at high rpms where ignition problems would not handle the loads.

ok4450: No light, no codes. MAF sensor needs to be plugged in or the engine doesn’t run. Learned that when I forgot to plug it back in once. Fuel - again, doesn’t seem to starve at hight RPMs. Fans come on as follows:

Primary (LH) or Dual Low-speed Fan(s) ON: 108 C 226 F
Primary (LH) or Dual Low-speed Fan(s) OFF: 105 C 221 F
Secondary (RH) or Dual High-speed Fan(s) ON 113 C 235 F
Secondary (RH) or Dual High-speed Fan(s) OFF: 110 C 230 F

Temps - not sure how it has affected the engine, but again, no coolant leaks internally or externally, so I didn’t blow any gaskets or crack anything as far as I can tell.

UncleTurbo: I’m thinking along the same lines as you - when the hose blew off, it blew hot coolant and water right into the Optispark ignition system. I already hosed off the engine (which I frequently do with water and Simple Green to keep it clean, with no seemingly ill effects). It’s basically distributor less in the common form - which is where the optispark comes in. Accel has a new unit for $300, factory units run about $400, and MSD wants $500 for theirs. I may try replacing the ignition coil as well (not a COP setup) and get a new set of plugs and wires. I’ll probably have to have my wife change the plugs and wires for me - that’s a tight engine compartment and my arms are too big to get in there.

I appreciate everyone’s feedback so far.

Does this engine have its distributor below the water pump? I can’t remember if this feature was offered on this engine.

Yes, it does. The optispark is a unique distributor system, in that it is attached to the front of the engine, with the crank running through it, and then the harmonic balancer/pulley on the very end of the crank. There have been horror stories of people having to replace them very early in their service life due to moisture getting in there and fouling things up. Since I have one of the last LT1s ever built (97+ 'Vettes and 98+ F-bodies had the LS series engine) they learned to vent the optispark to keep moisture buildup from causing problems.

The attached picture shows the optispark (yellowish thing on front of engine. You can see the splined shaft that goes into a coupling to drive the water pump.

Actually, if I have to replace the optispark, I may just drop another grand and replace the timing chain, balancer, coil and maybe the cam (if I had to take it all apart anyway, why not?) If I have to replace the plugs and wires again, I may just pull the manifolds and throw on a set of shorty headers too. Those plugs and wires are a real pain to replace. It may actually be faster to pull the engine to replace them and put it back in…

My area at the Chevy Dealer was not driveability but we all compared notes. The word on getting moisture (from leaking pump) into the distributor below it was to expect endless ignition trouble.

Right, and there were a lot of issues with that. My car has run strong since I bought it new in 97, and this is the first issue I’ve ever really had from the engine. I think I may replace all of the electrical components on the front just for good measure. I’d like to get 250k+ out of the block before I have to pull the engine open it up for the first time. Then it’s 383 stroker time.

I’m trying to decide which is more important - bathroom renovation or engine renovation, and I already know what my wife thinks…