projects and questions, new to me 1994

[conesare2seconds]

Hi everyone. Recently bought a high-mile 94 with a few needs and am putting together a project and maintenance list. Have a few questions, if you don't mind. This is my 9th C4 but first ZR-1.

- There is a knock on the passenger side of the engine. Poking around with a stethoscope seems to confirm it isn't in the engine proper, but it does conform to engine rpm. Has anyone solved a mystery noise in that area? I'm leaning toward exhaust noise (stock manifolds), maybe the PS cat substrate has broken up (a PS precat was the source of a similar knock on my old 91 L98).

- AC is inop. Compressor is supposed to have been replaced a few years and not many miles ago. It took a refrigerant charge, at first, with the AC compressor cycling. Now is showing system pressure but air at the vents is still warm. I took a tip from here and unplugged the forward pressure switch on the AC line that runs down the PS of the engine bay, and the engine cooling fans immediately came on (warm engine). Did I just diagnose a faulty pressure switch?

- On a related note, cooling is not up to par. Temps rise quickly when idling or in stop and go traffic in the current hot weather, but it stays really cool when moving. The dash gauge and the digital readout also disagree, with the gauge reading much higher than the digital readout. The plenum was off recently to replace several vacuum lines. I'm wondering about an air bubble. Make sense?

- Finally, when running up through the gears under moderate throttle, there is a miss around 3200 rpm in 2nd and 3200 and 4200 rpm in 3rd. The engine stutters momentarily, then resumes pulling. My guess is the stutter is related to the secondaries opening. With the engine off and key turned to run, the vacuum pump runs constantly, which I understand indicates a vacuum leak. Oddly, the car pulls to 6000 with plenty of power in the upper rpm range, so I'll say the secondaries seem to be activating fine.

Thanks in advance for your patience with a long post. There are a bunch of other small projects on the list, most of which I can handle from prior C4 ownership. Above are the main ones that are possibly LT5-specific. Any advice or suggestions are welcome and appreciated.
Tom

[Dynomite]

Welcome

Originally Posted by conesare2seconds

Hi everyone. Recently bought a high-mile 94 with a few needs and am putting together a project and maintenance list. Have a few questions, if you don't mind. This is my 9th C4 but first ZR-1. What is mileage? This then be your first LT5

- There is a knock on the passenger side of the engine. Poking around with a stethoscope seems to confirm it isn't in the engine proper, but it does conform to engine rpm. Has anyone solved a mystery noise in that area? I'm leaning toward exhaust noise (stock manifolds), maybe the PS cat substrate has broken up (a PS precat was the source of a similar knock on my old 91 L98). You say "knock" and usually Exhaust issues are a "tick" Is this with engine idling or under power or what? Is this forward or rear of engine?

- AC is inop. Compressor is supposed to have been replaced a few years and not many miles ago. It took a refrigerant charge, at first, with the AC compressor cycling. Now is showing system pressure but air at the vents is still warm. I took a tip from here and unplugged the forward pressure switch on the AC line that runs down the PS of the engine bay, and the engine cooling fans immediately came on (warm engine). Did I just diagnose a faulty pressure switch? I assume this is refrigerant R134A? When you charge with refrigerant you ground that pressure switch to make sure the AC compressor is engaged and compressing as you charge the Coolant System. The large AC line should get almost frosty when the coolant system is fully functional with the compressor engaged.

- On a related note, cooling is not up to par. Temps rise quickly when idling or in stop and go traffic in the current hot weather, but it stays really cool when moving. The dash gauge and the digital readout also disagree, with the gauge reading much higher than the digital readout. The plenum was off recently to replace several vacuum lines. I'm wondering about an air bubble. Make sense?
No on Air Bubble...Air bubbles get washed out quickly as long as the water pump is NOT AIR LOCKED in which case Temps would zoom up to HOT quickly....how recently was Plenum Off....I assume you drained coolant a bit which could AIR LOCK Water Pump.....but again...the coolant temperatures would go through the roof.

- Finally, when running up through the gears under moderate throttle, there is a miss around 3200 rpm in 2nd and 3200 and 4200 rpm in 3rd. The engine stutters momentarily, then resumes pulling. My guess is the stutter is related to the secondaries opening. With the engine off and key turned to run, the vacuum pump runs constantly, which I understand indicates a vacuum leak. Oddly, the car pulls to 6000 with plenty of power in the upper rpm range, so I'll say the secondaries seem to be activating fine. You have a BIG vacuum leak and your secondaries (including Secondary Injectors) are probably not operational ALL the Time.

Thanks in advance for your patience with a long post. There are a bunch of other small projects on the list, most of which I can handle from prior C4 ownership. Above are the main ones that are possibly LT5-specific. Any advice or suggestions are welcome and appreciated.
Tom

Check out my signature for additional help

As you describe your issues further you will find the boys and girls on this forum can solve each and every issue you might think you have

[Kevin]

if by "digital readout" you mean the one you can access by the secret hand shake it's in C not F

[RussMcB]

I'll take a stab at the easier one - overheating.

If the radiator has never been out, it is surely clogged with a shocking amount of leaves, dirt, etc. Removing the radiator and cleaning will help, but I think most (or many) members here have installed aftermarket radiators and completely solved the problem. It's not a quick and easy R&R, but worthwhile. I'm happy with my DeWitt's radiator and Jerry's Gaskets' hoses.

BTW - I bought an inexpensive '91 ZR-1 that needed work last year. I worked through the issues and now have a great, fun & reliable (super)car. :-)

[Racinfan83]

On the overheating issue - I had exactly what you described. I had Marc Haibeck pull the radiator and it was chuck full of garbage. Cleaned and re-installed, and along with the Haibeck chip that has both fans come on - mine never gets over 200 anymore. zr1specialist.com

[Billy Mild]

Honestly for the money that a new radiator costs at the auto parts store just replace it. I got mine for $110 from Advance Auto Parts. This was after I already cleaned my stock radiator and the endtank split as i was putting it back in.

Get a MityVac and test every vacuum line. Also make sure your check valve is still in good shape. I had 3 fail one right after another. It was annoying. That would make it so you wouldn't have control over your HVAC as well as the cruise.

[conesare2seconds]

Thanks for all the replies. This is the Dallas-area car a forum member was going to look over for another fellow, and for all I know he told him to run, haha. With its miles, mods and history, my ambition runs to enjoying it as a driver as opposed to NCRS Top Flight. I drove it 5 hours back to Little Rock and it rolled 176,000 my second day of ownership.

I'm getting the picture on the vacuum leak and will buy Marc's DVD and begin chasing it. Billy, I'll start with the check valve as the cruise is inop as you guessed. The plenum was off in the last few months and the prior owner replaced many vacuum lines and fittings.

I'll say the engine noise is a tap, like a collapsed hydraulic lifter. It is audible at idle and directly proportional to engine rpms. Without the stethoscope it seems to come from the middle of the PS between cylinders 4 & 6, or maybe closer to 6. It doesn't seem to come from the top of the engine, like the cylinder head or injectors/plenum, which is confirmed by the stethoscope. It seems "lower", coming from near the exhaust manifold. I tried to put the stethoscope on the block and nothing jumped out at me. The DS sounded the same as the PS with the steth on the block. I'll keep looking and will start with a cooler engine so I can get in close without as much radiant heat. The engine seems pretty healthy so I'm not in mortal fear (yet) of a significant internal problem,

I agree with everyone on the radiator. I once spent hours cleaning one with a pick to "save it" and am not sure I'd do it again for $110. That's the mist likely culprit, but I wondered if the cooling fan was a symptom. The digital temp display is switchable C to F, and it's on F.

The AC compressor short-cycled as I charged it and was definitely coming on, but after taking and apparently holding a small charge it stopped cycling.

Thanks for a great welcome to the registry forum.

[oricale]

Hey there, is this the red car from Gunbarrell TX??

[conesare2seconds]

Roger that, it is. Believe you had interest at one point?

[oricale]

yes I did!!, I'm sure you did very well!
I am very happy it went to someone who is going to enjoy it!

nice..post some pics when you can I have the Carfax if u need it