Also check to see if the fuel filter or fuel lines are on the headers... they should be secured to the A/C box and the frame rail on the passenger side...

If the fuel line is allowed to get hot with out fuel flowing through it (stagnent) then there is a chance the fuel can boil and cause vapor lock. This used to happen on carbed cars, but still very possible on EFI cars.

 

This is one that I'm absolutely positive of.... if the electric water pump ever fails because of a flaky connection, the analog gauge keeps climbing and the digital goes down. Also I've broken the connector off of the one in the head before (installing headers, go figure) and all it impacted was the analog gauge.

You really don't even have to do anything to test them short of make sure that when the car is cold that both your analog and digital gauges read cold when cold and hot when hot. If one of them isn't working, there's your problem. Well, A problem anyway.

 

OK, I did more testing and this is what I found.

Tested the N2O fuel cell and it works perfectly, removed the FP regulator to look it over really well, nothing visibly wrong. So, decided to take the car on a drive and check the coolant gages. Cold, car started immediately again. Let the car warm up and noticed that the dig coolant level was below the analogue by about 100 degrees. I thought that they would match as the motor warmed (as thermostat opened). After letting it get hot, the analogue gage was at 215 and the dig gage stopped at 193. I pushed the car to see if it would go up. The analogue did, but the dig never increased above 193.

So, I think the problem is with the coolant PCM. Does this sound like a failed censor?

 

Normally if the CTS is bad it will be default to 142*.

 

one sensor is in the head, another in the waterpump... 20* difference is tolerable.

 

Unfortunately the analog gauge is not linear, IE the gauge being halfway between 100 and 200 does not mean it's at 150. With this is mind there is no way to know exactly what the analog gauge truly reading. Also keep in mind that they are measuring the temperature at 2 completely different locations. The analog gauge measures from the cylinder head where all the heat is being generated, it is going to read hotter than the digital gauge.

Unfortunately that's not your problem, keep looking.

I still think it sounds like an ever so slight valve lash problem.

 

there is a way to know what is supposed to be reading....




Checking lash sounds like something worth investigating.

 

Great info, but that's for the digital gauge.

I was talking about the analog.

 

For any gauge, this doesnt help. But for the sensor, its everything! all you need is a thermometer and a digital voltmeter. This is the reference chart for the sensor.

 

True, however, because the gauges are starting out low and then reading hot as the vehicle warms up, it's probably not a sensor issue. If one of the gauges wasn't reading properly it might be different.

Don't get me wrong, it's great info and should probably be added to the tech sticky, but it's just not gonna solve this problem.

 

I had a chance to get the car out for another drive tonight and think I found a breakthrough. Indication piont back to the PCM coolant sensor. The car fired almost immediately while cold and the dig gage stoped at 193 degrees just like yesterday. After getting the motor hot (215 degrees analogue and what the dig gage indicated prior to the problem), I turned the car off and immediately turned it back on. Bam, fired right up again. I looked at the dig coolant level to notice it had dropped to 180 degrees. So, I turned the car off for 10 seconds and watched the PCM coolant level plummet. After 10 seconds, the gage read 75 degrees. Tried to start again and had the hard start again. I didn't have time to get under there and look around and hope to look it over Sunday. Maybe a broken, disconnected, or failed censor. Something is definitely up with it. What do you think??

I'm headed to the drag track for some fun tomorrow with the wife's 300C SRT8, so I will not have time until Sunday to look it over. I'll use the ohm chart to test the censor. I hope that it is just disconnected or something.... We'll see. Another thought, would the Meziere HD pump have anything to do with it?

 

If the digital gauge is erratic then it is gonna be a sensor or connection issue.

If your water pump ever quits your analog gauge will read WAY higher than the digital. If the pump comes back on it will equalize quickly. I dont think the pump is causing this problem but it is never a bad idea to confirm that it has good solid connections.

 

could be a bad sensor.... or sensor going bad.

Could be a pinched wire.

could be a low coolant/cavitation/aeration affecting the sensors readings.

Test the coolant temp sensor on the water pump
I say replace the sensor if it looks in any way like it was failing.. they arent too expensive.

you can use a resistor with a value corresponding to a temp frm that chart, and ground out one of the pins on the temp sensor connector to the block....this will test your wires.

 

1)If you are connected such that the pump and fans are running with the motor off it will cool off quickly.......but not in 10 seconds

2) I am kinda puzzled why the gauge would read correctly with motor running but drop with motor off???? A bad connection, faulty sensor, etc. would give bad readings motor running or not??

 

Gremlins don't care if it's running or not

 

Low coolant.... think about it... when running there is enough coolant moving around to get it on the sensor, when off, there isnt.

or a pocket of air/steam

 

I just had a similar problem. The connector for the temp sensor had been knocked off several times during the repeated installation and removal of an RD camber bar. I actually broke the plastic part of the connector off the sensor so I replaced it. I was at an autocross and the car starting really acting up. It would misfire and it was hard to start. I noticed the digital temp read out was all over the place. It seemed to display random readings.

Now for the interesting part; I finally just disconnected the sensor. The car was still hard to start but once running the digital temp read out would start at around 80 degrees and in 30-45 seconds it would progress to 193 and stop. If I turned off the car and immediately restarted it, the digital read out would do the same thing, start at 80 and stop at 193. The car would run pretty well. I was able to finish the autocross without much problem.

I bought another temp sensor from AZone that came with the connector. When comparing the new and old connector, it was obvious that repeatedly knocking it off with the camber bar had messed up the metal in the connector. I swapped connectors and the problems are gone.

So it seems the computer has a failsafe mode for dealing with a missing temp sensor, start at 80 and stop it after a certain amount of time at 193.

Andrew, before you get into the high dollar stuff, carefully check the sensor AND the connector. When I looked at my connector at the autocross it seemed ok. But when I compared it the the new one, it was obviously buggered.

Good Luck my C4 brother.

 

I had similar suspicion as Gas Junkie and had some time to take the N2O system out and inspected the coolant sensor. Well, I'm embarrassed to say it was disconnected! I had the exact same issue as Gas Junkie did and after taking things apart so much, the sensor connection was tore up, but the sensor is still good. I was able to connect it back and the car started immediately every time. To test, I disconnected and it began the hard start again (woofing sound and all). Connected it back and started immediately every time again.

WOW, I didn't realize something this small could cause so many problems! Anyway, I'm going to get a new sender and connector.

Thanks to everyone for all of the help!! It paid off.

 

Well that'll do it every time. Glad you got it figured out.

 

Glad you got it solved.

When you order the new part, you want the coolant temperature sensor. If you order the coolant temperature sender, they'll give you the sender that feeds the analog gauge and is located between the #6 & #8 exhaust ports.