The GTM Supercar Build Log

Archive for April, 2009


Thursday, April 30th, 2009

It’s finally officially alive. Took the GTM out for its first drive today. My wife was the honorary first passenger who also filmed it. The steering is very tight – I love it. Very responsive and quick. The torque is ridiculous – broke the tires loose a bit in 2nd just with a quick throttle blip.

Here are some vids and random pics from today.

First “Drive”, More Coolant, Exhaust Installed, Throttle Body Troubleshooting

Wednesday, April 29th, 2009

Today was pretty exciting. Before doing anything I checked the coolant from the other night’s fill; it had settled a quite a bit (ie: air pockets released). I decided to idle it for a bit and see how it cooled (or not). Before idling I threw on some wheels and finally got the GTM off its stands.

Since it was idling, and on the ground now, I decided to see if reverse worked. It didn’t. I realized it was because I didn’t set the gate cable properly to allow the cable to pull reverse, so I fixed that. With reverse fixed I backed out of the garage into the driveway, let it idle outside for a bit, then pulled it back into the garage.  It felt pretty good to be able to drive it a few feet! However, there is still air in the cooling system, as the temps got too high again after idling for about 15-20minutes.

With that out of the way I let the coolant settle for the afternoon and came back tonight to add more coolant. I added more the same way I did previously: disconnect the upper water pump hose, open the steam vents, open purge tank cap, and dump coolant into the water pump/radiator hose. Tonight I was able to add in quite a bit and am now at over 4.5 gallons (shouldn’t take more than 5 gallons). There are probably a few air pockets left to get out, so I will let it sit overnight again.

Also did some troubleshooting on the throttle body. With some adjustment I was able to get the car started with it (before it would throw codes and would only start with the scanner disabling them), however it would not idle. This seems to be a manufacturer defect, and not something I can fix, so a new throttle body hunt begins.

I also installed the exhaust.

Exhaust Mockup Install

Monday, April 27th, 2009

Taking a bit of a down time tonight so I just mocked up the exhaust (loose fit on headers, no O2 sensor plugs) to see how it would look, and to hear it a bit. The exhaust sounds very mean, and is still very loud.  I will take a video later!

Coolant Fill, Underbody, Throttle Body Troubleshooting

Sunday, April 26th, 2009

With the front skid bars in place I finished securing the front underbody panels in place with some self-tapping stainless screws. This will make removal of the underbody easy in the future, if needed.

After that I decided to fill the coolant system. The GTM takes around 4-5gallons of coolant, and the surge tank itself holds around a gallon of coolant. I had already filled the surge tank previously, as per the GM coolant fill guidelines. The GM procedure wants you to run the engine up to 210 degrees in cycles, so I had to hook up the coolant temp gauge first (easy). I popped the cap on the surge tank and idled for a minute, then got the temps up to 210 degrees. At this point the engine started to overheat (took about 10 minutes to get that hot just idling with some revving), so I shut it down. No coolant was being pulled into the system (air lock). To try to get the air out of the system I jacked up the front end of the car, opened the vents on the heads and popped the top radiator hose off of the water pump (not the hose going into the thermostat). I hoped gravity would suck the coolant in and get the air out, but no joy there. I decided to force the air out by dumping coolant (a 50/50 water+antifreeze mix) into the radiator hose I just removed from the water pump. To ensure that air could escape, I pulled off the radiator hose that runs to the surge tank. After a few gallons coolant started coming out of the radiator (good), so I capped that hose and continued filling until coolant started coming out of the steam vents. Capped those off again and kept adding coolant until all the air was out (the surge tank started to fill up). I then reattached the radiator hose to the water pump. I will cycle the engine later to get any remaining air out. Also, the cooling fans worked as they should, so that is one less thing to worry about.

After filling the cooling system I tried troubleshooting the throttle body issue. I removed the GTM throttle body TP sensor and put that on the Z06 throttle body and the gas pedal still worked. I then swapped the throttle body motors and still got the pedal to work. That tells me that the problem resides within the throttle body itself.

Idle Videos

Sunday, April 26th, 2009

Took some vids today of the GTM idling with open headers, and then with the high-flow cats on.


Sunday, April 26th, 2009

Today I decided to pull the TAC module out of my Vette to use on the GTM just to see if the TAC module was really the cause of the throttle issues I have been battling. Unfortunately, the TAC from my Vette solved nothing. The same non-responsive throttle greeted me. 

Tonight I returned and mindlessly poked around for a while hoping a ‘revelation’ would come to me.  I decided that I would swap throttle bodies with my Vette just to see if the new throttle body I am using for the GTM had a problem somewhere (most likely the actual throttle position sensor). I pulled the TB out of the Vette and put it on the GTM. Turned the key to ON and I had throttle response. Excellent! I decided since I had a proper working throttle I would try starting it up. Just a sputter and flame with a cam sensor error code. The code told me to reverse my cam sensor polarity (I had re-reversed it earlier in the week while messing around). I reversed it and the engine roared to life! Very rough idle, but it finally idled on its own. While I was basking in the glory I realized the MAF wasn’t hooked up, so the engine was running in speed-density mode. I hooked up the MAF and fired it up again – nice smooth (for a 624hp engine) idle!

Next up is coolant filling, running it through the gears, and maybe gokart ride!

Skid System, Wiring Loom, Hard-lines Attached

Sunday, April 26th, 2009

Since I am stumped on the engine/throttle issue I started on installing the skid system. This will protect the underbody of the car, as well as the actual body, from speed bumps, ramps, etc. Each skid bar (8 in all) is attached to the frame using tapped 10-32 holes. The process is painstaking, but the end result is worth it.  I attached the front four skid bars tonight and will tackle the rest once the engine is running properly and no leaks are found.

After working on the skid system I loomed up the oil pump wiring, and attached the hard coolant lines to the frame.

Transmission Oil Pump Wired

Friday, April 24th, 2009

Wired the oil pump up tonight. A keyed power source runs to the inline oil temperature sensor on the cooler inlet, which activates a relay that turns on the oil pump on when the sensor hits 180 degrees. The relay also grounds an oil pressure sensor that will trigger a dash-mounted led that will turn on when the pump reaches 15+psi.

Cable Shift Routing, Cooling Fan Grounds, Misc.

Thursday, April 23rd, 2009

Not too much going on right now with the build. I shipped off the TAC module to Fparts today so they can send me a replacement. All the diagnostics I have done point to a faulty TAC, so hopefully the new one fixes the problems!

In the meantime I fastened the cable shift gate cable to the frame and re-wired the cooling fan grounds. FFR’s instructions have you extend the black wire from the engine fuse panel to the black wire on the cooling fans, but in reality you need to ground the black wire coming out of the fuse box and also ground the black wire coming out of the cooling fan harness itself.

I started piecing together the underbody skid bar system. This will protect the frame (and body) and keep the belly pans on as well without having to rivet them. Easy removal of the underbody panels will be nice!

I also obtained a OBDII scanner from my friend Brian (THANKS!) which will help me diagnose things easier as time passes instead of having to wait for the loaner Tech2.


Monday, April 20th, 2009

It runs! Tonight with the help of the Tech2 scanner we figured out what the problems were. Now we just have to fix the problems. The main problem preventing the engine to start is that the PCM is losing communication between the Throttle Actuator Control Module and Throttle Position Sensor when the key is turned to off. This may mean the TAC is bad, or we simply need to add another constant hot wire to the ignition wiring when the key is in the start position. The camshaft position sensor is also triggering a code indicating the computer may not have been properly programmed to recognize the engine is using an LS2 cam sensor, and not an LS1 sensor. Regardless, it runs now, doesn’t really idle due to the TAC-TPS communication and cam position sensor issues, but it’s officially alive!