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New engine, starts, won't run
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
New engine, starts, won't run
Well after having my '92 LT1 on the lift for the last 3 months with a new powertrain going in, I was hoping to get it back on the road this week. After my tune arrived (finally) I tried to start it up - and no joy.
The engine has been rebuilt mostly stock on the lower end, with ported heads, a TB, injectors, hot cam, new G2 opti.
I can get it to fire with some throttle, but it quickly dies. The fuel rail shows ~47 psi held while cranking. I got my DSO on the coil primary for a quick sample before it died (great, at least its still under warrenty). I did see what looked like a good parade pattern with a burn line on each cylinder for the few seconds it ran. I also checked one cylinder at 140 PSI compression which seemed in line for my 7000+ ft elevation.
Any ideas??? Thanks in advance...
The engine has been rebuilt mostly stock on the lower end, with ported heads, a TB, injectors, hot cam, new G2 opti.
I can get it to fire with some throttle, but it quickly dies. The fuel rail shows ~47 psi held while cranking. I got my DSO on the coil primary for a quick sample before it died (great, at least its still under warrenty). I did see what looked like a good parade pattern with a burn line on each cylinder for the few seconds it ran. I also checked one cylinder at 140 PSI compression which seemed in line for my 7000+ ft elevation.
Any ideas??? Thanks in advance...
Last edited by scranage; Jun 13, 2006 at 01:56 PM.
Team Owner
Joined: Aug 2000
Posts: 40,156
Likes: 45
From: San Pedro CA
well... spark, fuel, air.
the basics.
with a known bad coil (if I am reading this correctly), I would check the coil harness. They are known to wear and not connect correctly.
Also, it sounds like the IAC needs to be relearned.
the basics.
with a known bad coil (if I am reading this correctly), I would check the coil harness. They are known to wear and not connect correctly.
Also, it sounds like the IAC needs to be relearned.
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
Not much to go on but how about the TPS voltage?Another thought is the IAC not opening.Sounds like your getting spark so check the basics.Either flooding or idle air issue is my guess.
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
Funny you should mention TPS, I had one code at first strat and it was TPS sensor. I figured I need to slot the sensor since I have a BBK 58MM TB, but the code vanished the last time I reset and checked. I'll look into it again.
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
Good news, bad news.
Looks like the TPS reference voltage is 1.5V, not 5V. I unplugged the TPS module and sensor and it still reads 1.5, so I guess I'm in need of a new ECM. Does that sound about right???
Melting Slicks
Joined: Apr 2002
Posts: 2,546
Likes: 0
From: Orlando FL
As the others stated, check the basics. When i rebuilt my motor last year, I had no issues upon starting. It did run rough a lil at first, but after taking the steps to reallign the IAC's, all was well. Idle came down ect. Good luck! Hope you get that baby back on the road soon.
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
Progress!!!
TPS reference voltage was definitely part of the problem. I managed to get a LM7805 voltage regulator crammed into my bad ECM and generate a 5V reference voltage for the TPS sensor & module. It's a temporary fix, but it starts better, and almost runs at idle now. I now think I need to relearn the IAC, but the manual only tells how to do it with a Tech 1 and brake module, which I don't have. Is there any other way to do this????
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Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
I have read several posts saying a 58mm TB is un necessary especially since you do not have a radical rebuild.I would pu the stock one back on.It may fix all your problems.There is a way to do the IAC with out a tech1 but I can't remember it right now check the tech tips.Mabey Bogus can lead you to the IAC setup.
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
There are a lot of opinions on the 58mm TB, but when I started shoping head ports, I got the 58TB on list of associated upgrades to get the rated power out of the head work, so I went with one.
After I get it running well enough to get to the dyno, I'd intended to run against both stock and the 58 with a final tune and see if there is any difference. I'd rather go back to the stock just because getting the duct on is a PITA, and at my elevation, I tend to doubt it helps to go with the larger bore.
Also, I didn't mention the cause of the TPS reference voltage failure. I had put the electric WP relay coil on the ignition circuit where it comes into the TPS module. I knew better to do this without putting a diode across the relay coil first, but put it on temporarily without one, and I didn't get away with it at all. The back current apparently got into the reference voltage circuit in the ECM through the TPS module - where both circuits meet somehow, I never found a schematic for inner workings of the TPS module. Anyway, the voltage regulator I added to the ECM fixed that for now, and a used ECM on Ebay cost me $230. Expensive lession considering that I already knew better. WTFO
After I get it running well enough to get to the dyno, I'd intended to run against both stock and the 58 with a final tune and see if there is any difference. I'd rather go back to the stock just because getting the duct on is a PITA, and at my elevation, I tend to doubt it helps to go with the larger bore.
Also, I didn't mention the cause of the TPS reference voltage failure. I had put the electric WP relay coil on the ignition circuit where it comes into the TPS module. I knew better to do this without putting a diode across the relay coil first, but put it on temporarily without one, and I didn't get away with it at all. The back current apparently got into the reference voltage circuit in the ECM through the TPS module - where both circuits meet somehow, I never found a schematic for inner workings of the TPS module. Anyway, the voltage regulator I added to the ECM fixed that for now, and a used ECM on Ebay cost me $230. Expensive lession considering that I already knew better. WTFO
Last edited by scranage; Jun 14, 2006 at 07:47 AM.
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
It does seem your TB is the problem here is a link on the IAC
https://www.corvetteforum.com/techti...D=94&TopicID=2
I know I have read better write ups on it though something like putting it into service mode and removing it etc. etc. hope someone who has done it will chime in.
https://www.corvetteforum.com/techti...D=94&TopicID=2
I know I have read better write ups on it though something like putting it into service mode and removing it etc. etc. hope someone who has done it will chime in.
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
I still haven't really made any progress even after going back to the stock TB, and trying to get the IAC to re-learn. It will fire right up without any gas now, but it dies after about 3 seconds. If I juice the throttle, I can keep it running a little longer, but not too much.
I've confirmed the fuel pump is running until quite a few seconds after it dies, so it looks like the oil pressure switch is keeping it on like it should. I'm not sure what to look at next. I'm open to any ideas...
I've confirmed the fuel pump is running until quite a few seconds after it dies, so it looks like the oil pressure switch is keeping it on like it should. I'm not sure what to look at next. I'm open to any ideas...
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
OK, I found another problem with the wiring on the MAP sensor this time. I now have it idling, but only with a little throttle, So it looks like I still have a problem with the idle air motor position. Is there any alternative to the Tech 1 to get this thing set properly???
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
Here is some more info might help
http://forums.corvetteforum.com/show...=LT%21+IAC+set
http://forums.corvetteforum.com/show...=LT%21+IAC+set
There is a MIN air adjustment too.You do have the FSM don't you?
http://forums.corvetteforum.com/show...=LT%21+IAC+set
http://forums.corvetteforum.com/show...=LT%21+IAC+set
There is a MIN air adjustment too.You do have the FSM don't you?
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
Redeasysport - Thanks for sticking with me on this! I do have the FSM, but haven't run across the minimum air adjustment on the IAC. On my 92 FSM, they only address the IAC adjustment with a Tech 1 as far as I have seen. I'm still digging, but if you have a pointer, I would appreciate it.
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
I believe it has to do with adjusting the screw on the TB for min air at least on older l98s but I am not sure about LT1s and do not want to mislead you.Try PMing CFI and see if he can confirm it.The pintle adjustment is all that is needed for the IAC I believe but the memory is not as good as it used to be.Good Luck
Last edited by Redeasysport; Jun 16, 2006 at 10:27 PM.
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
More good info
http://forums.corvetteforum.com/show...T1+minimum+air
another on L98
http://forums.corvetteforum.com/show...t=LT1+idle+air
Lt1 idle adjustment?
http://forums.corvetteforum.com/show...t=LT1+idle+air
http://forums.corvetteforum.com/show...t=LT1+idle+air
another LT1 idle problem
http://forums.corvetteforum.com/show...t=LT1+idle+air
Let me know if any of this helps
http://forums.corvetteforum.com/show...T1+minimum+air
another on L98
http://forums.corvetteforum.com/show...t=LT1+idle+air
Lt1 idle adjustment?
http://forums.corvetteforum.com/show...t=LT1+idle+air
http://forums.corvetteforum.com/show...t=LT1+idle+air
another LT1 idle problem
http://forums.corvetteforum.com/show...t=LT1+idle+air
Let me know if any of this helps
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
Book 2 pg 6E3-C2-13 shows a breakdown of a stock TB.Now I can't say for sure and based on the above links and some others that some tweaking of the idle air may be needed especially on a bigger TB since you have modded the engine.Anything after this is just suggestions and not known facts
If you jump pins 1&2 of the ALDL it puts the car in "service mode" and full extends the IAC to close it.Unplug the IAC and it will stay there then remove the jumper.Try and start the engine.If all is good it should run at base idle ~450-550 rpm.If you have to use the throttle a litlle to keep it running I would think you need to adjust the Idle stop screw to get it to run(#15 is a plug on it remove it to get to the screw) on the drivers side rear of the TB.Becareful to keep track of how much you adjust it in case it does not help so you will be able to return it to the starting set point.This is from my 93 FSM and your page may be different.I hope this solves your problem.
If you jump pins 1&2 of the ALDL it puts the car in "service mode" and full extends the IAC to close it.Unplug the IAC and it will stay there then remove the jumper.Try and start the engine.If all is good it should run at base idle ~450-550 rpm.If you have to use the throttle a litlle to keep it running I would think you need to adjust the Idle stop screw to get it to run(#15 is a plug on it remove it to get to the screw) on the drivers side rear of the TB.Becareful to keep track of how much you adjust it in case it does not help so you will be able to return it to the starting set point.This is from my 93 FSM and your page may be different.I hope this solves your problem.
Thread Starter
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
Progress!!
OK, I did all that, and did about a full turn in to get it to hold an idle. After it ran a few minutes, it smoothed out, and I backed the adjustment amost all the way back out and it seemed to run well at around 700 RPM using the IP tach, which I thought I recalled being calibrated too high. Anyway, after reconnecting the IAC harness, I was once again unable to get it to idle when I restarted it, and had to turn the screw in again where it had been before. Now I have an idle that hunts between about 700 and 1000, again, from the IP tach so I don't have an accurate way to tell for sure.
Any idea on what the hunting idle is about???? Anyway, I'm going to take it around the block and see how it drives. It's been since March when the tear down started, so I'm anxious to see if this thing still rolls, or if just sits on the lift.
Any idea on what the hunting idle is about???? Anyway, I'm going to take it around the block and see how it drives. It's been since March when the tear down started, so I'm anxious to see if this thing still rolls, or if just sits on the lift.