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Discussion Starter #1
97 Z28 M6, fresh build on the engine. Had some issues related to the crank position sensor. Car went through multiple heating cycles ( fans kicking on every time ). Finally fixed that, but had to replace the water pump during the process because it started leaking out the weep hole. I then had a misfire code. Replaced the bad fuel injector. Code went away.

Drove the car for the first time tonight and it overheated. Didn't sound like the fans were running, but I couldn't see them from the top and couldn't get under the car with it not jacked up. Check engine light for misfire came on again too ( P0300 ) after about 15 startups. It was when I raised the hood to check this out that I heard the coolant boiling ( despite being under 260° ).

Which sensor reads the temp and tells the fans when to kick on? If it's not the one on the front of the water pump, what does that one do? It was all the way in on the old pump, but wouldn't go in all the way on the new one. Got real tight with about half the threads still showing. Would that effect it's reading since there'd be a pocket of water there that isn't circulating?

Also, is there a place on the car I can run a jumper wire to real easy to force the fans on? I'd like to cool it down while running to avoid damaging the engine. I hate to just shut it off and let it cool on it's own. Especially if I'm going to have to get it up to temp to test the fans kicking on. If they don't, I'd like to manually force them on to cool the engine.
 

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Sensor on water pump tells pcm what coolant temp is. Pcm, in turn, provides the ground for fan relay coil. When this ground is applyed, fan relay points close and fans turn on.

You need to put car on an obd 2 scanner with real time data capability. See what temp the water pump coolant sensor is sending to pcm. Compare it to gauge reading. Factory fan turn on temps are 236 degrees for low speed, & 246 degrees for high speed.

97 fan system uses 3 fan relays. So you might want to troubleshoot them. Info at www.shbox.com

And you can download the 96 service manual at the following link. 96 fan system will be same as 97.
www.mediafire.com/?40mfgeoe4ctti
 

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Forgot to add, did you bleed air out of system after changing wp? Having air in coolant will cause overheating. Location of bleed ports on shoebox site.
 

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Discussion Starter #4
It must have had air in it. I suspected the fans weren't coming on, but it was hard to tell. Can't really see 'em from the top and the car wasn't up on jackstand/ramps for me to get under to see from the bottom. I couldn't hear them, but the engine was making those gurgling/boiling water sounds and it was hard to hear over that. I topped it off tonight and let it warm up and the fans kicked on and the temp came back down. It cycled just fine. I did use both bleeders just as I did on my 96 and 95 cars a couple of years ago and they never overheated. Somehow, I must've done it wrong, but did it right tonight.

Now I have another problem I'm hoping your expertise can help me with. I had the P0300 code immediately after replacing the CKP. My guess is it always had the misfire, I just wasn't getting the code due to bad sensor. It seemed to run pretty smooth. Turned out I had a bad fuel injector. Cylinder 2 wasn't working. Probably never did. I replaced that, cleared the code, started the engine multiple times ( about 10 ) and the SES light never came on. Started it another 5 or 6 times at church showing the car to my friends who've been following the project. Still no SES. Temp never got over 210 at the longest idle time.

Then my wife and I left church in it. We live literally 1.5 miles away. In that time, it got hot enough to boil the coolant. I wouldn't think it was hot long enough to blow a head gasket, but that's where I could use your expertise. It was the SES light coming on as I was backing in the drive that made me realize something was wrong. I popped the hood ( it was code P0300 again ) to look to see if I can see anything amiss. That's when I heard the coolant gurgling.

In your experience, with such a short trip, is it likely to be a blown head gasket or some other problem that just took 15 or so startups to light the SES and it's just a coincidence that they happened at the same time?
 

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Make sure the steam pipes are hooked up at back of reads and are routed to the correct place. Shoebox site shows steam pipe hook up.

Make sure you got all air out of system. It doesn't take long for coolant to overheat with air trapped. The reason coolant system needs bleeding is because radiator is lower than engine. People who have mid engine cars are use to the air bleed routine.

Is your air dam intact under the front of the car?

P0300 can set if there are problems with crank hub. Check that out.

Tests for blown head gasket include compression check, leak down test, and if your good with a vacuum gauge, it can detect blown hg.
 

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Forgot to add, another reason for P0300 is bad optic wheels in optispark. Especially common with parts store, ebay cheapies, & other China made ones.
 

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Discussion Starter #7
Steam pipes are hooked up correctly.

I thought all the air was out. Front of the car is elevated, opened both bleeders. Closed them as coolant would come out. Continued to fill. Not sure why it overheated. I replaced the coolant that boiled out and used the bleeders again ( with car up on ramps ) and it didn't overheat today.

Air dam is intact. The drive home was so short, I don't think it would've mattered. Probably didn't get up fast enough to shut off the fans ( if they were even coming on ).

I know how to test for blown head gasket, especially in this case since the fear would be it blown between cylinders ( only explanation of misfire but no leaks or fluid mixing ). I was only asking if such a short overheat session would typically cause that. Since it's hard to get to all the plugs in these cars, compression tests aren't a quick and easy thing to do just to check it off the list.

As for the crank hub... What should I check for? What exactly would be wrong with it to cause the code? I know it's tight. Torqued it to 73 ft/lbs just like the Hayne's manual says.

I do have an eBay opti on the car, but I put an original mitsubishi sensor in it. After fixing the dead injector, the code was gone. Didn't come back until the car got hot. I'd think a bad reluctor wheel in the opti would have set off the code from day one.

Can the coil or ICM cause the misfire code if they are going bad? I know some people have ICM troubles associated with heat, but they usually die at operating temp. This car didn't mess up until it went well beyond operating temp. It also didn't die, just started missing ( this time it is noticeable and not smooth like before ). They also usually go back to working fine after the car cools down. This misfire was still present on cold startup 24 hours later. The only reason I ask about the coil and ICM is because that's the only part of the ignition system that isn't new.
 

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Crank hub being tight eliminates it from problem list. Unless something is keeping it from moving in as far as it can go. The crank hub serves as the magnetic field that crank position sensor gets it's readings from. A crank hub that is too far out on crank, can give faulty readings on sensor.

Many times a very hot engine will miss. Can't say exactly why, but my guess would be a combo of over heated electronics & engine rotating part clearances that are now too tight.
 

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Discussion Starter #9
But would the miss still be there after the engine cools off? Unlike with the dead injector, it's got a bit of shake to it now. So it's missing worse than when it was running on 7 cylinders. So I know it's not a false reading. I checked the live data, but I honestly don't know what I'm watching for. I don't know what I should be seeing to know what would be off.

Going by the manual for troubleshooting, it says to look at the snapshot info to see which cylinders are misfiring. Some kind of list of how many misfires per cylinder. I couldn't find that listing anywhere in the snapshot data. I don't know if it didn't record it or I just don't now where to look.

I had assumed I got the P0300 because it was missing on multiple cylinders. But I only had once cylinder down as it smoothed out and the code went away when I replaced that one injector. Do you have any idea why I didn't get code P0302 indicating it was that one? Because if it would let me know which cylinder is misfiring, it would make it easier to narrow down.
 

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Discussion Starter #10
I have an adjustable spark tester, but I don't know these cars ignition voltage to know how to gap it. As soon as I find that out, I'll test the coil to make sure it's not just a weak spark affecting all cylinders.
 

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Some of the troubleshooting tests in the factory service manual, assume the tech is using a GM tech 2 scanner. It does more than aftermarket scanners can do.

If you haven't a clue to what the scanner data means, PM me with yoyr email address. I'll send you the pdf file of " scanner readings and what they mean".
 
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