454 TBI truck backfires on cold start for about 30 seconds

[Chevy3+3]

Well this morning I turned on the ignition and let it sit for about 60 seconds before cranking it. After it fired up I was waiting for the usual backfires to start but to my suprise......it never backfired. Last nights low temp was only 55 degrees so it may have not been cold enough and may be why it didn't backfire.

[rich weyand]

So that may be it.  Full pressure to the fuel rail and O2 sensor to temp. 

Or it may have just been too warm overnight to have the problem.

Keep us posted.

[Chevy3+3]

I am the guy that had the backfiring problem on an 88' Chevy Crew w/454 TBI 4x4 Dually only when the overnight low temps were in the 40's or below. Truck has 130k miles and doesnt use coolant and doesnt burn oil. Since then I have remedied several small issues and changed a few parts but I still have a problem. My fuel mileage is roughly 8-9 MPG normal driving and highway speeds of 55mph. At 55 my engine RPM is about 2400-2500. I have no Service Engine light on after the truck is started.

I believe I am running to rich when the overnight temps get in the 40's. I would get intermittent backfiring for about 20-30 seconds and then the truck would clear up and run smooth the rest of the day. When started the truck idles up to about 1200 RPMs when cold. When warm it idles about 1000 RPMs and about 700 RPM in gear when warm. These RPMs appear to be normal.

Here is a list of what I have done:

1. Replaced the Coolant Temp Sensor
2. Double checked that the EGR valve is working correctly
3. Found and fixed all the vacuum leaks (including replacing TBI baseplate gasket)
4. Checked timing (bypass wire unplugged) and found it was set at 0 BTDC
5. Set initial timing at 4-5 degrees BTDC (bypass wire unplugged)
6. Unhooked battery for a few minutes, plugged bypass wire back in and hooked battery back up.

When I started the truck this morning with a 45 degree outside temp the truck fired up and after about 5 seconds of ignition the truck started popping/backfiring a few times. It did not do it as long as usually. I walked back to the tail pipe and it smelled a VERY strong fuel odor. I almost think it might be running to rich.

I do have a cracked passenger side exhaust manifold. I have a new replacement and plan to install it within the next few days although I do not believe this to be the reason I am backfiring.

I believe something is telling my TBI to dump more fuel causing me to run rich when cold and potentially even when warm. Should I or would it be in my best interest to replace the O2 sensor? There is only one O2 sensor and it is located on the drivers side exhaust manifold and is hooked up with a single wire.

I have also thought about checking/resetting the valve clearance or preload making sure they are correct.

Any thoughts or advice would be greatly appreciated. Thanks

[81_Chevy]

have you check the ECM to see if its throwing any codes? I'm sure that would help alot..

maybe take it down to a local parts store and have them check for codes.

[454Man]

What is...Cracked exhaust manifold for 500 Bob. I've seen where they cause lean condition in cold a weather

What's under the hood??? If you have to ask maybe we shouldn't race...

[Chevy3+3]

have you check the ECM to see if its throwing any codes? I'm sure that would help alot..

maybe take it down to a local parts store and have them check for codes.

I was under the impression that as long as I didnt have any SES light on the dash then I wouldn't show any codes?

[Chevy3+3]

What is...Cracked exhaust manifold for 500 Bob. I've seen where they cause lean condition in cold a weather

What's under the hood??? If you have to ask maybe we shouldn't race...

Can you elaborate? Are you saying that possibly the cracked manifold is showing the ECM a lean condition causing it to deliver more fuel than necessary?

Thanks

[81_Chevy]

have you check the ECM to see if its throwing any codes? I'm sure that would help alot..

maybe take it down to a local parts store and have them check for codes.

I was under the impression that as long as I didnt have any SES light on the dash then I wouldn't show any codes?

honestly, your guess is as good as mine lol. im not great when it comes to EFI or TBI. But since most places do it for free i figured why not.

[bd]

I was under the impression that as long as I didnt have any SES light on the dash then I wouldn't show any codes?

The SES light illuminates when a detectable problem occurs, then typically remains lit until the ignition is cycled Off.  Any resulting trouble codes are stored in the ECM memory for subsequent retrieval.  If the problem doesn't recur within a certain number of ignition On-Off cycles, the code is automatically purged from memory.  So, if there has been any recent 'code' activity, a trouble code may be stored.

However, a complication can occur that muddles efficient diagnosis if any monitored sensors have been unplugged while the ignition was switched On, setting false codes.  Therefore, the best approach is to: 1) retrieve any existing codes and write them down, 2) clear the ECM memory by removing the ECM-B fuse or disconnecting the battery for 20+ seconds, then 3) drive the vehicle normally to see if any new codes occur.

Can you elaborate? Are you saying that possibly the cracked manifold is showing the ECM a lean condition causing it to deliver more fuel than necessary?

An exhaust leak can allow fresh air to be drawn into the exhaust stream.  If the leak occurs ahead of the O₂ sensor, the sensor will register the added air; the ECM will interpret the O₂ signal as a lean condition and command extra fuel, causing the engine to run rich.  However, an O₂ sensor must be above a temperature threshold of ~600-650° F in order to function correctly.  Hence, O₂ sensor output is ignored by the ECM during cold startup, until the sensor is sufficiently heated by the exhaust flow.  Later model 4-wire O₂ sensors have special integrated heating elements that bring the sensors up to temperature rapidly, but your single-wire sensor doesn't have that attribute.  So, the O₂ sensor is not likely an issue for you.

First, fix the exhaust leak.  Then you are back to a prior post...

• What does the fuel pressure measure?
  
  
• Are there any stored trouble codes?
  
  
• Remove the air cleaner and wipe off the tips of the injectors with a rag.  Turn the ignition ON, but do not crank the engine.  Do you hear the fuel pump run for about 20 seconds?  Check the injector tips for fuel - they should be dry.  Repeat this cycle 2 - 3 times.  Injector tips should remain dry.  This step determines whether the injectors are leaking fuel.
  
  
• Refer to the factory manual and check the operation of the Air Management (aka: Diverter) Valve and air injection system - on a cold start it should inject air into the exhaust for about 30 seconds or so to heat the cat.
  
  
• Check the MAP and TPS functions using a voltmeter.  You'll need a voltage vs vacuum chart for the MAP.  Regarding the TPS, you're mostly interested in the throttle closed voltage (~0.56 v) and a smooth voltage transition to full throttle (< 5 volts).

[454Man]

Thanks BD!  I couldn't respond in time

What's under the hood??? If you have to ask maybe we shouldn't race...

[Chevy3+3]

Thanks BD. My exhaust manifold crack is after the O2 sensor. I drip tested the injectors and they appear to remain dry. They also spray an even cone pattern and I see no drips I from injector while running.

What's controls the fuel & air mixture when the ECM bypasses the O2 sensor? From what you said above it is almost as if once the O2 sensor get heated up then it stops backfiring.

I also wanted to ask what would happen if the Coolant temp sensor was unplugged? Would this cause an full time rich condition or lean condition.

[454Man]

I just fixed my header collector gasket leak and all I can say us wow I was losing a lot of power. It also caused miss fires at idle and during driving.

What's under the hood??? If you have to ask maybe we shouldn't race...

[Chevy3+3]

I just fixed my header collector gasket leak and all I can say us wow I was losing a lot of power. It also caused miss fires at idle and during driving.

What's under the hood??? If you have to ask maybe we shouldn't race...

Well tomorrow I will try and resolve all my exhaust leaks. I have exhaust manifold to replace and a flange gasket on the other manifold to replace

[454Man]

Hope this helps you out.

What's under the hood??? If you have to ask maybe we shouldn't race...

[bd]

Thanks BD. My exhaust manifold crack is after the O2 sensor. I drip tested the injectors and they appear to remain dry. They also spray an even cone pattern and I see no drips I from injector while running.

What controls the fuel & air mixture when the ECM bypasses the O2 sensor?  From what you said above it is almost as if once the O2 sensor get heated up then it stops backfiring.

I also wanted to ask what would happen if the Coolant temp sensor was unplugged?  Would this cause an full time rich condition or lean condition.

The ECM applies a constant ~0.45 volt across the O₂ sensor.  The O₂ sensor is designed to modify that ECM voltage.  But, until the O₂ sensor reaches >600° F, its effect is null and ECM O₂ voltage remains constant at 0.45 volt - and the ECM operates in "Open Loop."  Essentially, Open Loop is a program condition that simply excludes O₂ sensor output, while monitoring CTS, MAP, TPS and Tach to adjust fuel and timing.  However, once the O₂ reaches >600° F, it modulates the fixed 0.45-v ECM signal between ~0.1 v (lean) and ~0.99 v (rich), causing the ECM to enter "Closed Loop."  Closed Loop operation incorporates the O₂ sensor output to tightly control fuel delivery for a 14.7:1 air-fuel mixture.

The CTS varies its resistance from 100,700 Ω (-40° F), to 13,500 Ω (20° F), to 7,500 Ω (40° F), to 185 Ω (210° F).  In other words, very high resistance when cold.  Unplugging the CTS would force fuel enrichment along with an illuminated SES lamp and code 15.

Correct the exhaust leak then continue from there.  Now, would also be a good time to check the spark plugs, cap, rotor, and plug wires, if you haven't done so already.