Help with Exhaust problem!

[mrluhman]

I bought an 89 blazer recently that needed a new engine so I had to tow it back to Iowa from Minneapolis. The seller did mention that he had just put new true dual exhaust on the vehicle that set him back $500 right before the engine failed. This Blazer is all stock and in great shape so I bought a new GM crate 350 to put in it. When I got it back $3100 later it ran just ok and not as good as I had hoped for a new engine. I took it back to the dealership becuase the check engine light kept coming on at high speeds (65 and over) but it never failed or showed any signs of distress. Another $1200 later (new battery cable, new battery, multiple new sensors, ect...) it was determined by the dealership that since the exhaust had been modified with no catalyitic convertors the engine was getting no backpressure and that is what was tripping the check engine light. This vehicle is a daily driver for me but I am in the process of restoring it back to factory new with the help of parts I am finding on LMC truck and people here (thanks udy!). So the symptoms are as follows: it seems to surge and not idle evenly especially after highway speeds, it will hesitate when accelerating from a stopped position (traffic lights), and it gets terrible gas mileage. The dealer also said it has a small crack in one of the manifolds. They said I could get a chip that would stop the check engine light from going on or to just unplug the bulb. I am a purist and dont want to do either of those things. This is my 3rd Blazer (80,85,89) so I know how a well tuned factory 350 should sound and preform in one of these trucks and it just doesnt. My intention for this vehicle is daily driver and to preserve a factory standard piece of old Detroit. Would it be best to modify the exhaust back to factory specs, add the catalytic converters back on to the true duals, or is there an easier/cheaper way to make the engine run right without wasting all that money the previous owner had just made in the true dual exhaust system? This is the only Blazer I have owned with TBI so could it be a problem with that? I feel like I have been a bit gouged by the dealership here and I would like to put this problem out to fellow K5 enthusiasts before I throw more money to the dealership. Please let me know what I can do to make this 1989 perform like it was 1988 rolling off the factory lot?

Joe

 

[dyeager535]

Dealer is an idiot. Backpressure doesn't make power nor does lack of make problems.

Is the cracked manifold on the same side as your O2 sensor?

 

[Fabrimacator21]

Whats the cam like in the crate motor? It might be a little too much for the stock computer.

Unless you've got a serious exhaust leak I don't think exhaust is your problem.

 

[dfl701]

i have had two trucks without cats on them, by 83 blazer now and my 96 z71 with the vortec. the cats are not going to cause all the problems your having. the check engine light yes so to get that to go you need to put the cats back on. that just my experience with no cats though

 

[dyeager535]

His won't have any problem with the O2/lack of cats, as the OBD2 stuff (1996 and later) is the one that monitors catalyst efficiencty with rear O2 sensors.

Anything that happens "past" the O2 sensor in OBD1 doesn't affect the computer in the slightest, except plugged exhaust.

 

[mrluhman]

The manifold crack is small but it is on the same side as the O2 sensor that was recently replaced. Should i drop the coin for new manifolds?

 

[dfl701]

i was thinking of it running so poorly. People always blame cats, which sometimes it could be but with our trucks i dont see the big deal that would explain why my 96 had the error code though

 

[dyeager535]

Someone on here had the EXACT same issue (crack on the O2 side manifold) and the cure was a new manifold.

Don't have to get a new one, nor replace both, but to be honest, if I was going to do it, I'd just drop a set of headers in there if it came down to that or buying a new manifold. A used manifold is likely to crack again, and may require a spreader and/or more work to get to fit, while headers will require a bit of exhaust work to bolt up. However the power gain over manifolds would be substantial.

Of course, in your area, I assume rust is a problem, so headers probably won't last near as long as a new manifold.

I believe some places are now selling repro manifolds, so the cost may be reasonable. I'll never go back to manifolds, but I also don't deal with salt.

As an edit, SOME of your problems (mileage for one) are likely to be cured by getting rid of the crack. The O2 sensor is likely "seeing" air that is coming in that crack, and therefore it thinks the engine is running lean, dumping more fuel than it needs. This would be most apparent at throttle positions of cruise MPH and below.

Even if it's NOT the cause, it is most definitely an issue that should be fixed.

What code(s) are you getting? Also helpful to know. Very easy to pull them with a paperclip.

 

[Thunder]

Yes you should replace the manifold. beings it is cracked it could affect the 02 readings
BTW I would avoid your local dealer. They do not know what they are doing or talking about. Dual exhaust will have no effect on the 02 readings if you still have the stock manifolds. Un plug the CEL bulb??????? Stay away! Stay far far away from that dealer. That kind of crap is why GM is bankrupt.
Your problem may be a bad EGR valve. Bad idle, CEL at highway speeds points to Bad EGR od EGR solenoid.
You need to find a good mechanic or learn to work on it yourself. Is not that hard to fix TBI yourself. It is a very simple fuel injection system. Plenty of info here and people who will help.