Ok I know that this is a well beaten topic and I have thoroughly searched this site and many others for an answer to the issue I am having with my Lowered 1995 Z28.

So for the last few months I have been battling an issue with my cooling system. I am experiencing the typical tempeture climbing above 200* while sitting in traffic and cooling back down to 160* when driving. Well here's the problem after driving the car for long periods I am experiencing engine detonating (knocking) while taking off from a start (6spd) and knocking restarts while running errands. While cranking the engine it is slower than if it were cold and when it fires off it knocks like it has overheated.

It was suggested that I replace the starter so I did that...no change! I replaced the thermostat, radiator cap (Oreilly's parts)....no change. After alot of frustrating hours looking over every part of the cooling system I had a break through. It blew the bypass hose off the radiator. Instantly it hit me *Blown Head Gasket*.

So upon tearing down the top end I find that the left head gasket had been leaking compression gasses into the cooling system. Ok Problem solved right....Wrong! After following every single step in performing the repair. GM gaskets, Machine shop reworked heads, Head Bolts, and step by step reinstallation the problem has not been fixxed.

Now Im getting frustrated! Today I replaced the radiator since its another piece of the puzzle and still not fixxed. I have bled the cooling system over and over again using Shoebox's methods, Verified cooling fan operation both high and low, replaced both temp sensors, have an airdam installed, cleared any possible blockage from the front of the condenser, Cat is not present in the exhaust. The only part i have not messed with is the water pump. One thing I have noticed is immediately after I start the car cold there is pressure on the cooling system. Is this normal for the reverse flow systems?

So to sum up my questions:

1: Why does my car knock when leaving a stop sign or upon start up immediately after driving for a long period of time?

2: Is cooling system pressure normal at cold start up?

3: Would doing the TB By-pass help eliminate air at the back of the heads?

Any help in answering these questions will be greatly appreciated. I have to drive the car to Idaho from California next month and need to make sure I have no issues. Thanks in advance!

 

Does not sound like you have a temperature issue. Not sure how you know there is pressure in the coolant system. Don't think there is any correlation to knock and ping and your coolant.

So the first question since you did all this work is have you done a test on the coolant to see if the HG's are leaking?

Have you had the cap off the radiator when it's at operating temperature to be sure there is flow?

Do you have the correct thermostat for an LT1 which is different than others?


Have you run a higher octane fuel to see what happens to knock and ping?

Sorry I don't have any answers, just questions.

 

Have you scanned the PCM for indications of knock, like knock retard, and rapidly incrementing knock count?

"Climbing above 200*" is not hot. It's exactly where GM intended the engine to operate, and the timing map is totally consistant with that operating temperature. The knock sensor is capable of pulling up to 15* of advance, to eliminate the knock. If you are truly experiencing detonation, there is something radically wrong, and it's not due to the operating temperature.

Are you sure it's spark knock (detonation), or could it be a bearing knock or piston slap? Until you scan the PCM's indicators of knock, and its response, you don't have enough info.

Slow cranking of the engine may indicate too much spark advance. Are you runnning the stock tune?

When "the machine shop reworked the heads", did they shave them? Did they do anything that would cause an increase in compression ratio - e.g. - thin gaskets?

 

Excellent point. CR increase.

 

Yup Im definately an idiot...lol! The problem was caused by the low grade fuel I have been running since the outrageous fuel increase, but when I started to notice the knock was when I was experiencing the over heating issue. Since I repaired the head gaskets and the cooling system it did change some but in my mind it was still a cooling sysytem issue. Well I added a bottle of Lucas Octane booster hit the freeway and then a jaunt through the hills behind my house and low and behold problem solved.

When the heads were rebuilt they did take a little off and I used GM head gaskets. Will I possibly have an issue later from the thin gaskets?

Thanks for helping me clear my mind and start using logic again. I have now restored the confidence in driving my car the 1000 miles next month.

 

Also if you put in a 160* t-stat and don't reprogram the computer to change the fan on temps they will not come on to cool it until the factory temp which is 220* & 226*, I believe.

 

226°/235°

 

Just how low did you go on octane? The knock sensor/PCM knock retard system should eliminate audible knock before you can hear it.

 

I was half right

 

I had been running 87 octane for about 5 months. Before that I was running 93 or 100 octane depending on my mood.

As for the Tune I beleive it is bone stock, but I could be wrong. I have driven a few Z28's and mine definately has alot more power and torque.

The knock was very audible when warm but the knock was only present when under load leaving a stop in first gear and restarts after warm up. Other than this issue the car runs very smooth and has lots of power. Now that I have raised the octane rating the knocking has subsided to where I dont notice it at all.

 

Another excellent point. I never used anything but low octane fuel and never had to worry about knock. You might want to investigate why you heard it since you should not have.

This might seem like a stupid question but did the tops of the pistons and the combustion chambers get cleaned of carbon when the HG's were replaced? Years of carbon build up will make an engine knock and ping. But as stated before, the PCM knock system should have dealt with it.

 

I didnt need to clean any carbon from the pistons, but the knock was there prior to the tear down.

Im just going to keep driving the car since its not knocking anymore and if it acts up again I will post up.

For the Bay Area Guys I will have the car at Boston Market on the El Camino tomorrow afternoon for the weekly meet and greet. If you have the time come on down and hang out.