Hard Start in 96 454. Please Help, this is a doozy

[454cid]

I've read that exhaust leaks will mess with O2 sensors, because they get extra oxygen that way. I guess because the exhaust is pulsed the pressure isn't continuous, so some ait goes in the leak, otherwise it wouldn't make sense for air to be going into a pressurized system.

 

[98_k1500]

Thats more myth than truth. o2 sensors don't actually "sense" oxygen. They are just exhaust gas temp probes. Not really sure how they got the name o2 sensor in the first place...

 

[brycebba]

I have also read posts that say that a bad exhaust leak can mess with the o2 sensor reading and report a lean condition.

 

[98_k1500]

^^^^ actually, it goes the other way. If a substantial amount of exhaust is exiting the pipe before the sensor, it will see a drop in temp. This will make the sensor think its rich. A lean condition would be signified by a hotter exhaust. Keep in mind these o2 sensors have a pretty limited range too, they only read after the engine is up to temp. You can have a vacuum leak enough to make a sensor read out of range and the engine still run.

 

[98_k1500]

They can be somewhat affected by an exhaust leak tho, It would have to be a MAJOR leak (broken pipe or something) tho before anything drastic happened. Definately would not be noticably weak or run bad. Bad mpg at the most.

 

[brycebba]

The exhaust does get a lot hotter than my wife's Tahoe... A lot. Could the pcm try to adjust if the cats are plugging and generate a lean condition.?

 

[brycebba]

Didn't get much running time with the new cats on but it isn't any better despite the increase in flow. I didn't drive it at all, just started it. This thing sucks

 

[brycebba]

I gave up and took it to a mechanic. The mechanic has taken it on as a challenge at this point because it has him stumped. "on a 1-10 scale of pain in the ass your truck is a 15" is what he said about it. He has gone over everything I have done while he has had it the last 5 days and he said everything looks good and all parts test fine. Compression is awesome. He is thinking that it is possibly a bad cam.

 

[88GMCtruck]

Wow, I'm sorry to hear your having issues still. I'm in a similar boat, truck runs great other than the hard start... I wish you luck.

 

[brycebba]

Thanks man, sometimes trucks are a pain.