Check Engine Light on and Different exhaust tone

[Kocher93]

So I've had my 89 blazer for about a year now, and for the longest time I could remember every once in a while say....1 in 50 start ups my check engine light will be on and my exhaust sounds like theres no cat. It will be really deep and way louder. There also seems to be more power like this aswell. Anyone know what this could be? And if It does happen I can shut it off and turn it back on and itll be gone.

 

[K85 Octane]

does it have more power cause it sounds cooler?

can you get the code? That would go a long way with trying to figure this out

 

[Kocher93]

It's seriously such a rarish thing to were itll happen at like midnight or comming home from a party or morning. But no it just feels like theres more to it, it feels like the exhaust thumps way harder

 

[Blue85]

I would assume it to be an exhaust leak. The code is probably O2 out of range. The extra power could be from running extra rich under this condition. The mystery is why it is intermittent. EGR valve? Could the catalyst be broken and moving around?

 

[British Slave]

It's seriously such a rarish thing to were itll happen at like midnight or comming home from a party or morning. But no it just feels like theres more to it, it feels like the exhaust thumps way harder

I wonder if it could be excessive amount of alcohol in the fuel???

 

[Cornfield creations]

Smog pump going out? That wouldn't cause the engine light to come on, but I had an 86 with a 305, the smog pump was bad and it sounded like a JD 4020 when idling. Took off the belt and it stopped the noise, and it ran better.

AFA the engine light coming on that could be only so many things on a OBDI system. They don't have nearly as many parameters as a OBDII system.

 

[smier]

I would assume it to be an exhaust leak. The code is probably O2 out of range. The extra power could be from running extra rich under this condition. The mystery is why it is intermittent. EGR valve? Could the catalyst be broken and moving around?

I think Blue85 is on the right track. You need to check for codes, plain and simple, a sensor or something is acting up and causing the ECM to not go into closed loop, it's just running off of a preprogrammed bunch of settings, which will make it seem to run great, but will kill your fuel mileage. Might as well start figuring it out now, because it will most likely become more of an issue down the road. The cat shouldn't make a difference on his truck, the ECM wouldn't even know if it was missing, there is only one O2 sensor on these, and it's before the cat. Plus, most of the time when a cat is bad, it usually causes poor running, not the feeling of more power. See if you can pull a code, then check all your sensors and their wiring, probably wouldn't hurt to unplug each sensor and replug them in paying attention to the connectors and the wiring. It may just be an intermittent poor connection that is just starting to act up on occasion... These OBD1 systems are simple, and work great if you learn how to use them as they were designed, but if you attempt to just throw parts at the problem without diagnosing it, they can cost you a ton of wasted money.

 

[smier]

Something to keep in mind with the OBD1 systems, it's been around for almost 30 years, it has it's limitations, and because of that, it's possible to get multiple error codes because of one problem. For example, on my '86 TPI camaro I had a MAF(Mass Air Flow) sensor code and an O2 sensor code causing the check engine light to come on and the car to run like crap sometimes and good other times... Many guys would run to the parts store, and buy a new O2 sensor($30+-) because it is much cheaper than the MAF sensor($300+used). However, the O2 sensor was actually ok. The real problem was with the MAF sensor. It was coated in oil from an overzealous previous owner over-oiling the dual K&N filters. $7 can of MAF cleaner and the problem was solved. The MAF sensor giving out bad readings was causing the O2 sensor to go out of it's acceptable range. Long story, but this is the type of thing that has given the OBD1 systems a bad reputation, guys Have thrown parts at things that weren't the actual problem because they didn't know any better.