LT1 backfires when left in sunlight
Thread Starter
Instructor
Joined: Jan 2010
Posts: 106
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From: Coral Springs Florida
The cts sensor seems fine its is about the same as outside temp when starting engine cold. It thens climbs smoothly up to operating temperature. I bought flow matched injectors from Jon at FIC 22# (92 lt1). Fuel pressure is around 38# at start up. I dont have a MAF in 1992 but I do have a MAP.sensor would that cause an issue when its hot out on start up
Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
closest electrical item to the underside of your hood is the ECM.
Could you put a damp cloth over it, roll it out into the sun and try that, as an experiment? I feel that the ICM/coil is too far down to be affected by the sun's heating of the hood. Especially on a cold engine situation.
Could you put a damp cloth over it, roll it out into the sun and try that, as an experiment? I feel that the ICM/coil is too far down to be affected by the sun's heating of the hood. Especially on a cold engine situation.
Thread Starter
Instructor
Joined: Jan 2010
Posts: 106
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From: Coral Springs Florida
Thats a great idea ! I will try that tomorrow afternoon but I thought when the engine is hot in stop and go traffic the ecm would also be really hot but its definitely worth a try !
Heel & Toe
Joined: Mar 2012
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Just my two sense... I'm by no means an expert. My first thought is that it's fuel related.
While I'm not incredibly knowledgeable on the Vette fuel system in detail, I'd guess that you have a pressurization issue when the sun heats up the fuel in the tank or lines (think someone mentioned the fuel rail/injectors earlier). Who knows, take off the gas cap and see if you've excessive pressure. I've actually had the "you gotta be kidding me" experience from the gas tank not venting from a faulty gas cap. Worth a shot... it's cheap and easy.
While I'm not incredibly knowledgeable on the Vette fuel system in detail, I'd guess that you have a pressurization issue when the sun heats up the fuel in the tank or lines (think someone mentioned the fuel rail/injectors earlier). Who knows, take off the gas cap and see if you've excessive pressure. I've actually had the "you gotta be kidding me" experience from the gas tank not venting from a faulty gas cap. Worth a shot... it's cheap and easy.
Drifting
Joined: Aug 2009
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The OP says he can start it in a garage no problem. He also says that he can push it out of a cool garage, immediately start it and it backfires.
Complete shot in the dark (pun intended), but the ONLY thing I can think of that is 'sun sensitive' is the light sensor in the dash (near the windshield) for the a/c system. Could it somehow be causing electrical havoc elsewhere? Bizarre possibility for a bizarre issue. Maybe cover up that light sensor and see what happens!
I'd certainly look to get that EGR code resolved as well.
Complete shot in the dark (pun intended), but the ONLY thing I can think of that is 'sun sensitive' is the light sensor in the dash (near the windshield) for the a/c system. Could it somehow be causing electrical havoc elsewhere? Bizarre possibility for a bizarre issue. Maybe cover up that light sensor and see what happens!
I'd certainly look to get that EGR code resolved as well.
Last edited by jmgtp; Mar 13, 2012 at 11:27 AM.
Race Director
Joined: May 2001
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The OP says he can start it in a garage no problem. He also says that he can push it out of a cool garage, immediately start it and it backfires.
Complete shot in the dark (pun intended), but the ONLY thing I can think of that is 'sun sensitive' is the light sensor in the dash (near the windshield) for the a/c system. Could it somehow be causing electrical havoc elsewhere? Bizarre possibility for a bizarre issue. Maybe cover up that light sensor and see what happens!
I'd certainly look to get that EGR code resolved as well.
Complete shot in the dark (pun intended), but the ONLY thing I can think of that is 'sun sensitive' is the light sensor in the dash (near the windshield) for the a/c system. Could it somehow be causing electrical havoc elsewhere? Bizarre possibility for a bizarre issue. Maybe cover up that light sensor and see what happens!
I'd certainly look to get that EGR code resolved as well.
Drifting
Joined: Aug 2009
Posts: 1,971
Likes: 243
ah, you're right:
"If I roll it out of the garage even with a cold engine and leave it in the sun it will act up."
there was about zero chance that could have been the problem anyway! Hard to think of what gets hotter from sitting in the sun vs when the engine is up to temp. The interior of the car would certainly be sweltering from sitting in the sun, vs when the engine is running and windows down/ac on.
"If I roll it out of the garage even with a cold engine and leave it in the sun it will act up."
there was about zero chance that could have been the problem anyway! Hard to think of what gets hotter from sitting in the sun vs when the engine is up to temp. The interior of the car would certainly be sweltering from sitting in the sun, vs when the engine is running and windows down/ac on.
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Race Director
Joined: May 2001
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Yeah this one is bizarre. You would think that if it's an ECM that was getting too hot it would:
1. Act up with engine heat
2. Continue to act up with the combination of sun / engine heat after startup
Kinda makes me lean towards super heated fuel rails or something like others have mentioned.
It's kinda a lot of work, but here's one way to rule out the ECM, unplug it and take it with you. Seriously, park it outside in the sun, pop the hood and disconnect the battery cable and then just pull the ECM completely and carry it inside. Close the hood and let the car bake in the sun all day. Come back out, plug up the ECM, hook up the battery and try it. If it still gives you trouble, it's not the ECM. If it solves it, it is the ECM.
1. Act up with engine heat
2. Continue to act up with the combination of sun / engine heat after startup
Kinda makes me lean towards super heated fuel rails or something like others have mentioned.
It's kinda a lot of work, but here's one way to rule out the ECM, unplug it and take it with you. Seriously, park it outside in the sun, pop the hood and disconnect the battery cable and then just pull the ECM completely and carry it inside. Close the hood and let the car bake in the sun all day. Come back out, plug up the ECM, hook up the battery and try it. If it still gives you trouble, it's not the ECM. If it solves it, it is the ECM.
Burning Brakes
Joined: Apr 2006
Posts: 1,218
Likes: 7
From: Burleson Texas
Just my two sense... I'm by no means an expert. My first thought is that it's fuel related.
While I'm not incredibly knowledgeable on the Vette fuel system in detail, I'd guess that you have a pressurization issue when the sun heats up the fuel in the tank or lines (think someone mentioned the fuel rail/injectors earlier). Who knows, take off the gas cap and see if you've excessive pressure. I've actually had the "you gotta be kidding me" experience from the gas tank not venting from a faulty gas cap. Worth a shot... it's cheap and easy.
While I'm not incredibly knowledgeable on the Vette fuel system in detail, I'd guess that you have a pressurization issue when the sun heats up the fuel in the tank or lines (think someone mentioned the fuel rail/injectors earlier). Who knows, take off the gas cap and see if you've excessive pressure. I've actually had the "you gotta be kidding me" experience from the gas tank not venting from a faulty gas cap. Worth a shot... it's cheap and easy.
, this was my first thought. The evap purge can on older gm vehicles can fill with fuel... the vettes tanks do get alot of exposure when its warm out. Maybe you are building pressure in the tank and its sending it though the evap and sending fuel towards the engine. Or the pressure in the tank is feeding your pump some air + fuel.
Do you get a fuel smell when you lift the filler door for the gas cap? Is there a large 'whoosh' sound when you remove the gas cap?
Remove the gas cap next time you let it sit out in the sun. Then try it out and see if thats the issue.
I highly doubt its heat related to anything on the engine itself... the sun doesnt get to 200* like your engine does while its running....
Thread Starter
Instructor
Joined: Jan 2010
Posts: 106
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From: Coral Springs Florida
First I just want to thank everybody for helping me with this weird problem. I have noticed a lot of air pressure in the tank when i take cap off to fill it up with gas. I thought that was normal because our Ford trucks also do that . I will park it with the fuel cap off in the driveway for a hour and see what happens . Like I said it only does it for 30 seconds and I cant tell if its weak spark related or just loading up on fuel in such a short amount of time. It also does it with the top down with not much interior heat buildup.
Heel & Toe
Joined: Mar 2012
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Well heres my initial thought: Not being a qualified mechanic and only being a member here for about 12 hours... I didn't feel much qualified to give an opinion. I must be maturing because it never slowed me down in the past.
The tank and gas would absorb heat, but more critical is the pressure increase from the tank and gas expanding. If the cap is not venting the excess pressure, it's possible that the pump would take some time to neutralize the pressure in the tank at start up but might shortly after by means of the return line. The resulting pressure would initially upset the pumps output volume as the pump is not designed to pump with significant pressure on the input.
Your fuel line pressure may indeed read ok, but it might be a mix of fuel and air as the pump might be cavitating because of the pressure on the input side of the fuel pump. Fuel and air mix to the fuel injectors would wreak havoc on the ignition process.
Like I said, I ain't a mechanic, but I get thrown in the fire everyday to fix things I know little specific information about. I just plug along till I get it undickered. More aptly put, I fake it till I fix it.... and then explain it away!
The tank and gas would absorb heat, but more critical is the pressure increase from the tank and gas expanding. If the cap is not venting the excess pressure, it's possible that the pump would take some time to neutralize the pressure in the tank at start up but might shortly after by means of the return line. The resulting pressure would initially upset the pumps output volume as the pump is not designed to pump with significant pressure on the input.
Your fuel line pressure may indeed read ok, but it might be a mix of fuel and air as the pump might be cavitating because of the pressure on the input side of the fuel pump. Fuel and air mix to the fuel injectors would wreak havoc on the ignition process.
Like I said, I ain't a mechanic, but I get thrown in the fire everyday to fix things I know little specific information about. I just plug along till I get it undickered. More aptly put, I fake it till I fix it.... and then explain it away!
Thread Starter
Instructor
Joined: Jan 2010
Posts: 106
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From: Coral Springs Florida
Today I got a late start in the afternoon and it was not that hot out when I was able to try things out. First thing I did was take off the fuel cap and it did release some back pressure then I parked it outside for about an hour. When I started it up the exhaust popped 3 or 4 times in a row then smoothed out. So I put gas cap back on . Then I put some frozen cool packs in a towel and put them on top of the computer and let it sit for about an hour. This time it started up with no popping and very smooth . I was so happy then I noticed it was only 78 degrees out and overcast. Hopefully I will be able to try again tomorrow afternoon with the frozen packs on the computer and it will be hot and sunny out. Its going to be a long night for me waiting to try again .
Burning Brakes
Joined: Apr 2006
Posts: 1,218
Likes: 7
From: Burleson Texas
Have you performed a leak down test on your injectors?
If there is a slight amount of pressure in the tank, say 4psi, and you have a leaking injector, this could pump a little bit of fuel into the engine.
The ecm gets much warmer than the outside temp while the engine is running. I highly doubt that is your issue... but test it anyway!
If there is a slight amount of pressure in the tank, say 4psi, and you have a leaking injector, this could pump a little bit of fuel into the engine.
The ecm gets much warmer than the outside temp while the engine is running. I highly doubt that is your issue... but test it anyway!
Thread Starter
Instructor
Joined: Jan 2010
Posts: 106
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From: Coral Springs Florida
I did perform a leak down test after I installed the injectors I bought from FIC . These are a 22lb flow matched set for my stock 92 LT1. Still had the same problem on start up. I also had a fuel pressure gauge hooked up one time to see if the pressure was off when the engine was backfiring. It was at 38lbs when it was acting up and the FPR was not wet with fuel.
Thread Starter
Instructor
Joined: Jan 2010
Posts: 106
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From: Coral Springs Florida
I wanted to update everyone on my LT1 intermittent backfiring on startups. Like I said before it only did it during the first minute after starting engine then ran great. I heard a couple of you guys saying ICMs do weird things so I looked up ICMs on youtube. I found a video from duralast that explains everything about ICMs. It said the ICM works off of the crank sensor all by itself for about the first 2 minutes only then the computer helps take control of ICM as well with a bypass wire. The backfiring only occured during the first 2 minutes Then the computer kicked in and no more backfiring. It also said never replace the ICM without replacing the coil at same time. I bought a new AC delco ICM and coil and installed them with the heat transfer paste on ICM. The paste on old ICM was gone and the MSD coil Had 2 white powder spot burns on opposite sides next to the square metal surrounding it. The coil must have been shorting out on startup to leave those powder burns. I dont know if that was caused a bad ICM or just a bad MSD coil. The good news is I have put on over 400 miles last month and no more backfiring at all. I can now go to car shows without anymore embarassing c4 explosions through the mufflers !!!!!!
Instructor
Joined: Nov 2009
Posts: 164
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From: Spring Texas
Change your MAP sensor. I've chased some strange problems in my 95 LT-1. On advise of a local speed shop, I changed the MAP sensor (use only ACDelco) ACD # 273-3205, GM # 1906672. After replacement several strange an un-error coded problems disapeared. I took the old one apart and the heart is a small gland that expands and contracts with change in atmospheric pressure aganist a pizzo electric resistive device that changes the input 5 volts to a range between .3 and 4.8 volts. The old MAP gland was as hard as a rock from heat. Because ACD-Amazon had sent me the wrong sensor originally and refused return, I then took it apart (new) and the gland was pliable. The speed shop owner, who runs an LT-1 in his 52 Ford pickup learned about the impact of a failed MAP sensor the hard way. If you look at the non 5 volt terminal with a VOM the voltage will range with engine RPM as you pull the TB open. With a 92 I would change it without measuring as I have not seen an operational MAP on any LT-1 I have worked on. They all deteriate with heat. Good Luck