454 TBI truck backfires on cold start for about 30 seconds

[Chevy3+3]

Since the weather is turning cooler and overnight lows getting in to 40's my truck the last few mornings has been backfiring out the exhaust pipes. It only lasts about 30 seconds and then the truck runs great the rest of the day. It never backfires again. If the overnight temps stay warmer then it doesn't backfire on start up.

Could it be an EGR problem or a coolant temp sensor problem causing it to run lean?

I just don't see how the cooloant could warm up in a matter of 30 seconds?

Any thoughts? Thanks

[pholliday1]

We got a couple of things here. First if its back firing, generally that's to much unburned getting down the exhaust pipe meaning a rich condition. Secondly if the computer is "Faked out" thinking its to lean and creating a rich condition the next step is to start trouble shooting o2 sensor coolant sensor etc. If you need help with that let us know. There are a couple TBI Guru's on the site.....I'm not one of them.

[Chevy3+3]

I found a broken vacuum tube that went to the Air Injection Valve ao I fixed that. I made sure the EGR valve was working properly and not stuck. I also replaced the coolant temo sensor on the intake. I then fired the truck up and the backfiring remains but once again it stopped after after 30 seconds.

I then got some carb cleaner and started spraying the throttle body base gasket and spacer gasket. I found that when I sprayed it on the passenger side the engine bogged some and/or slowed down. So I clearly have a TBI gasket leak. I think that might be my problem.

[Engineer]

Keep us informed with what you find.

[zieg85]

Keep us informed with what you find.

x2   30 seconds may be all it takes for the gasket to expand from the heat enough to run fairly decent...

[pholliday1]

Sounds good, be wary of buying and "throwing parts" at the problem. Keep your money in your pocket, there is a way to troubleshoot everything on your motor with enough patience you'll get to the bottom of the problem. There's a lot of smart people on this site. Good Luck!!

[Chevy3+3]

Well I replaced the TBI gasket between the spacer and the intake manifold. Once I got it removed I could clearly see where it had been leaking. I put it back together and started the truck. It only backfired or popped  about 4 times. Definately not as long as the previous mornings. I wonder if the TBI needed to retune since I got rid of the vacuum leak through the TBI base plate.

I hope it fixes it but I'm sure I cant be that lucky

[rich weyand]

What other vacuum leaks might you have?  Hoses all OK?  Power steering booster diaphragm OK?  No unused open vacuum ports?

Like you say, though, could just be that the ECM will have to re-learn the car.

[Chevy3+3]

All those vacuum hoses appear to be hooked up and not leaking. I will know shortly if it going to backfire. Getting ready to go start it for the morning

[Chevy3+3]

Well unfortunately my popping & backfire still remains. It backfired about 5-6 times this morning in a time frame of 20 seconds or so. Even after it stops popping if I get into the throttle it seems to stumble just a little like its clearing it throat. I checked exhaust when it was clearing out and it was a little black telling me its a little rich or had loaded up a little.

Could my timing being off a little off affect it only when its cold out? I'm almost sure it is something simple but hard to figure out the simple things sometimes

[rich weyand]

Well, if the mixture is wrong on a TBI, it seems to me it is either the 1) sensors, 2) ecm, or 3) injector(s).  The most likely remaining culprit here is probably the O2 sensor; you already changed the coolant temp sensor.  Not sure how to troubleshoot the O2 sensor.  Anybody know if the O2 sensor for this setup has a heater circuit?  If that's out, the O2 sensor will read wrong until the exhaust gases bring it up to temp.  How many wires does your O2 sensor have?

[bd]

Here are a few things to check:

• Does the SES light come on with ignition?
  
  
• Are there any codes?
  
  
• Check the fuel pressure.
  
  
• 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.
  
  
• Refer to the factory manual and check the operation of the Air Management 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.

[rich weyand]

Here's something you might try.  When you start the engine cold, turn the ignition on, but do not start it for 60 seconds.  Put your seat belt on, check your mirrors, light a cigar, whatever.  That'll let the fuel come up to full pressure and the O2 sensor heat up.  Then start it and see if it acts the same.  If that fixes it, you can then see how long you need to leave it on before you start it.  Maybe it only needs thirty seconds to get itself ready.

[Chevy3+3]

The truck only has 1 O2 sensor that I can find and it on the drivers side manifold. It only has 1 wire to it. The truck only shows the SES light when the switch is on and the truck not running.

After the truck is done popping then it runs great. Its gotta be fuel & air related I think

[Chevy3+3]

Well I'm not sure what to try next. I think I have exhausted myslef looking for vacuum leaks. If I had a decent vacuum lesk I think I would see it run bad even when warm. But this still only remains on the first start up of the day when the outside air temp is cooler. So in my mind it has gotta be either the cool intake air, the coolant temp sensor (which I already replaced) or another sensor whos duties are involved with cold starts (choked starts for colder weather). 

I will try and turn key on in morning and leave sit about 60 seconds before cranking and see if there is any change.

I forgot to mention that on a cold start. It starts and idles up like it should and then within 5-10 seconds is when the popping starts. It then only lasts about 20 seconds. The colder it is the longer it lasts.

Today after it was warmed up the RPMs were 1000 when idling and today was 75-80 degrees. Seemed a little high to me but may be normal.