Dan's ZR-1 Update Blog
Random updates on my 1991 Corvette ZR-1 project.
December 15, 2006
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It's been a long time since I updated my ZR-1 page with project progress. I decided to start this blog so I can update progress easier and hopefully be more consistant about doing it.


I'll start with the engine work that I have done the last two summers. When I first got the car it suffered from a few of issues.


 First: From the time the car was started until it reached full operating temperature, oil temps above 200 degrees, the car was a beast, lots of power. After reaching full operating temperature it would loose power.


Second: The car had a small drip oil leak coming off of the rear of the oil pan. There would be a table spoon sized oil spot the next morning after running the car. Also I noticed oil sitting in the crevices of the injector housings.


Third: The car had an intermittent idle problem after warm-up. Upon coming up to a stop when the clutch was depressed sometimes the rpms would drop down to about 450 for a moment and then come back up and setttle in at the normal 600 rpm idle. Once in a great while the car would die out during the rpm drop, it would be worse if the A/C was on. It would start right back up when restarted.



Fourth: The secondary vacuum pump was not coming on when the car was first started.


As you can see from the picture at the top of this post at least part of my oil leak was coming from somewhere in the valley. I was greeted to quite a mess when I pulled the plenum off. It's pretty common for the oil pan bolts to loosen up on LT-5's, I found mine was no different so I tightened all of the oil pan bolts to the torque specification in the service manual. Also the pan plug was loose and leaking as well, so that was also tightened to spec. When I took the plenum off I found all of the bolts for the breather box cover were very loose, this is also common from what I understand. These were all re-tightened to specification. Another major contributor to the oil mess was the the crankcase vent hose that goes across the back of the valley and tee's into the PCV's. It had become soft and blown at least three large holes, I am sure oil was spraying from there as well. This part is also discontinued, White Racing makes a real nice reproduction that I replaced mine with for about $120.00.


The secondary vacuum pump turned out to be dead so I replaced it with a brand new one from the local dealer. I tested the secondary actuators and they worked to specification. I noticed that they were put in backwards and turned them around. However once I turned them to the correct orientation they did not work smoothly and hung up so I turned them back around to incorrect. I guess they were too used to being backwards.


I removed all of the fuel injectors and had them tested and cleaned. The secondary injectors were failing after they got hot so I tossed them and got replacements. The primary injectors tested fine after being cleaned so I put them back in. I replaced all of the o-rings. I got the new Multech injectors from the local auto parts house where I get a substantial discount. It cost me $320.00 for the 8 secondary injectors. The primary injectors were not working all that well before they were cleaned so this might have contributed to the idle drop problem. The secondaries not working when hot would explain the loss of power after getting to operating temperature.


I did not test the plug wires but I replaced them as long as I had the  plenum off.


I thoroughly cleaned everthing under the plenum and zip tied all of the vacuum connections. It looks much nicer under there now than the picture shows. I also cleaned up and painted the plenum, the throttle body and the air horn.


After re-assembling everything I tested the secondary vacuum system and found no leaks. I changed the spark plugs, gapped to .050" and started her up. The idle drop problem was much better but still there to a very slight degree. The power loss problem at heat soak was better but also still there to a lesser degree.


I replaced the MAP sensor and the idle air control with no discernible change. I regapped the spark plugs to .035" and this helped allot with the idle drop problem. I also used my friends vacuum leak detector and found I had vacuum leaks around the plenum gaskets. So I removed the plenum, cleaned up the sealing surfaces and put fresh gaskets on. I should have done this the first time around. The idle drop problem seemed to go away. Also when I regapped the plugs my flywheel knock seemed to go away as well.


I replaced the custom prom that came in the car with a custom prom from Marc Heibeck. Marc changed the idle up to 900 rpm, changed some things with the air temperature sensor to get rid of the heat soak power loss, eliminated the power switch, got rid of the header backfire and a few other things. When I put the chip in the heat soak power loss was gone and the idle drop seemed to be totaly eliminated. All was well for the summer of 05.


Now this last summer a new interesting problem has come up. Actually it started in the spring. When the engine is cold and you first run after starting for about 10 minutes it has a lean stumble up until about 2400 rpm when it takes off like a banchie. It was minor when it started and got progressively worse as time went on. I decided to change the plugs and it went away with the plug change and then started up again after awhile and progressively got worse. If I let the car sit in the driveway and warm up for 10 minutes after starting it runs fine, no lean stumble. It is only if I try to drive it in that first 10 minutes after starting.


So, as soon as I get a chance I am going to remove the plenum and change the primary injectors, I purchased a brand new set, and change the coils to new ones. I think the coils are original to the car. I probably should have changed the primary injectors when I changed the secondaries. Hopefully this will solve the problem. I also purchased a new ECM in case that needs replacing.


So that pretty much brings me up to date with the motor stuff. Tomorrow I will post an update on the rest of the car.


2006-12-15 19:38:04 GMT