Backfire, why?

[Paul Higg]

Since it has gotten cold here in Georgia I have had backfires. Here is what happens. I keep it in the garage. I start it and back out the temp is 68 degrees on the speedo. So I drive through the subdivision and have to get on a road that is 55 or faster so I gotta get up to seed fast. If there is no traffic I can accelerate slowly and it is fine but if I punch the gas it is sluggish and I hear what sounds like backfiring. This only stops when I reach a temp of 140 degrees. Is this normal or do I have a problem?

New opti, plugs, wires, water pump, overhauled throttle body, new TPS, fluids okay.

Thanks.

Paul

 

[c4c5specialist]

Hi there,
Are all the parts OEM???
Coil wires are known to arc intermittantly.
Allthebest, c4c5

 

[hotshotzny]

since it has gotten colder out my 84 has been making some odd noises also.. but only when its warming up.... i wouldnt call it backfireing but while it is at idle on very cold mornings i get a lot of loud popping from the exhuast.. but it stops once the car warms up and gets going down the road

 

[Paul Higg]

Yup, all OEM. It only happens when the engine is cold. As soon as it warms I can punch it and I am pinned back into my seat--a great feeling--and all is right with the world.

Paul

 

[Paul Higg]

since it has gotten colder out my 84 has been making some odd noises also.. but only when its warming up.... i wouldnt call it backfireing but while it is at idle on very cold mornings i get a lot of loud popping from the exhuast.. but it stops once the car warms up and gets going down the road

That's EXACTLY what I get! I wonder if this why a lot of northern Vette owners put them up for the winter, or is it because of icy roads?

 

[c4c5specialist]

HI there,
If your coming off the throttle within maybe 2 minutes of start time, it could just be the air injection working. Or the air injection check valves could be weak. Your still in open loop, so your O2 sensors are not helping fuel trim calculations.
If its after that, maybe something else.
Allthebest, c4c5

 

[Paul Higg]

Coming off my throttle? It happens during open loop but maybe lasts a little longer. It did not do it during the summer. It only does it now that it is cold and only until the engine reaches 140 degrees. After that it is fine. It takes a few minutes for it to reach 140 degrees so I do not really know how long it will be in open loop. How long is it in open loop? I will have to time it today as it is 32 degrees out. But I do know it is fine after 140 degrees. So the question is is it a closed loop problem or is it normal until the engine warms up?

Thanks for you help, I will copy it into a document to keep it for information.

Paul

 

[rascal_rascal_99]

Mine did basically what you're describing when I first got it. It would hesitate and backfire some occasionally when it was warmed up, but worse before it warmed up good, my problem turned out to be a bad O2 sensor.

 

[Paul Higg]

Well, I can look into it but it NEVER does it when it is warm.

Thanks.

Paul

 

[TedC]

Paul, I know yours is a 1994 LT1, but the FSM for a 1989 L98 lists these posible causes under backfire
1. Connector or air duct at MAF
2. EGR being open all the time
3. EGR gasket
4. Spark plugs for cross fire (check cap, wires, and routing)
5. Ignition system
6. Ignition timing
7. Fuek system
8. AIR system check valves
9. Compression
10. Valve timing
11. Output voltage of igition coil
O2 sensor isn't used until you are in closed loop.

 

[jmccloud]

Cold sputter

Paul,
Have you checked for codes? It sound like a faulty ECT Engine Coolant Temp sensor (the one that plugs into the water pump) and that may throw a code.
140F is about right for closed loop. There are some threads that describe which pins to connect in the ALDL connector to reveal open/closed loop but they're for the older connector - your year has an OBD II connector even though the computer is OBD I. I looked but could not find the pins for your year. When the two OBD I ALDL pins are connected, the service engine light blinks at different rates for closed than for open loop. Check post #11 in this searched thread http://www.corvetteactioncenter.com/forums/showthread.php?t=89928&highlight=open+loop
Another way to tell open/closed is to monitor the instant mileage when under steady load, not accelerating. The mpg will increase when it goes to closed loop. You might observe the instant mpg under steady load while warming up. When the mileage increases, check the temp to see if it's 140.
Good luck and let us know what it is when you find the solution.

 

[Paul Higg]

WOW! Great advice guys.

Let me tell what I have done since the summer.

Replaced opti, plugs, wires, removed and cleaned the EGR, new EGR gasket, cleaned the MAF with some very gentle spray, replaced the fuel filter, replaced the ECT sensor, ALL GM OEM parts.

I like your ideas and I am going to look into them. Also, I try to pull codes once a week and I have no codes.

I have a lot of stuff to troubleshoot with now and I appreciate it. I WILL keep you informed in this thread so that if I get it resolved it may help others.

Paul

 

[Paul Higg]

Paul,
Have you checked for codes? It sound like a faulty ECT Engine Coolant Temp sensor (the one that plugs into the water pump) and that may throw a code.
140F is about right for closed loop. There are some threads that describe which pins to connect in the ALDL connector to reveal open/closed loop but they're for the older connector - your year has an OBD II connector even though the computer is OBD I. I looked but could not find the pins for your year. When the two OBD I ALDL pins are connected, the service engine light blinks at different rates for closed than for open loop. Check post #11 in this searched thread http://www.corvetteactioncenter.com/forums/showthread.php?t=89928&highlight=open+loop
Another way to tell open/closed is to monitor the instant mileage when under steady load, not accelerating. The mpg will increase when it goes to closed loop. You might observe the instant mpg under steady load while warming up. When the mileage increases, check the temp to see if it's 140.
Good luck and let us know what it is when you find the solution.

I looked at the post number 11 but I have a numbered plug. For instance I jump pins 4 & 12 to pull codes. Which numbered pins would I jump to keep it in open loop to look for that flashing light?

Paul

 

[Paul Higg]

Okay fellas, this is what I JUST did. I had to go to the grocery store so my wife and I took a stopwatch and started the timer when I started the engine. It was 46 degrees outside when we left. I look at the instant MPG too and it kind of seemed to be reading low. So the car was driving real lousy, it had hesitations when I punched the gas, poor performance, seeming stalls, and what sounded like a muted backfire but not an actual backfire out of the tail pipe but that sound coming from under the hood. Now, I haven't looked up the description and operation of the open/closed loop system yet so I do not know how long it actually takes. But when it hit 140 degrees it ran perfectly. Also, the stopwatch was at more or less 2 minutes and 40 seconds. Before I started it I tried to pull codes but I got none.

Is this a little more descriptive? I need more information about the open and closed loop function. I think it might be that OR it could be that the engine doesn't like cold weather. It wasn't doing it when it was warm outside. OR, it may have nothing to do with the cold weather but it suddenly cropped up. This is puzzling to me but I am sure we will get to the bottom of it.

Now, to bury my nose in the FSM for a while.

 

[Paul Higg]

Well, I did a search and found an excellent description of how the car operates in open loop/closed loop. It looks like I might have a problem in open loop. Now, can anyone help me with this? What would cause this problem in open loop? What components could possibly cause it? I know it is working fine in closed loop.

I forgot to mention that I also have a brand new adjustable TPS. I adjusted it a few months ago and it worked perfectly. Could it need readjusting? Is it even in the system in open loop?

What I need is a list of parts that are in open loop. I know the ECM is for one.

Thanks.

Paul