This is a page from a recent emissions test on a 1990 LS, 5 spd. No EGR on this model, AFAIK.

It has less than 70k miles, but was basically parked for several years before I started working on it. Engine runs ok at idle and at steady speed, and a little rough on acceleration. The cat converter is probably ok at this mileage, other emissions related components have not been checked yet. I am courious if anyone has a good idea of where to start looking, w/o throwing too many new parts at the problem.

The graph shows the engine runs way too rich on even moderate acceleration. HC emissions exceed the range of the test, and NOx is not far behind.

 

i would start with your o2 sensor, it very cheap at any autoparts store and you can do it yourself, that will help with it greatly, then run a fuel injector cleaner through it, that should help a lil bit as well, replace all your fuel filters i think that should cover it shouldnt be more the $30 bucks

 

Yes, pretty easy to get to, and I have a universal single-wire sensor ready. Not sure that would account for the rough engine under acceleration though...

Done the inj. cleaner already, plus some Seafoam straight into the intake.

Would a bad fuel filter cause this, even though it seems to be running rich?

 

yeah it can account for it in some instances, when was the last time you did a tune up? ( changed wires, spark plugs, pcv, etc )

 

Before I started to work on it, the car was parked for several years, except for now and then to run the engine.

Plugs are new. Cap and rotor looked almost new. Wires look good. PCV was removed and checked (I did buy a new one, but it does not seem to be a perfect fit into the grommet. The original one has a barb at the end, the new one was smooth). Vacuum lines look good. Valve clearance was adjusted per spec. Timing belt was inspected and retensioned at the same time.

Since the ecu is pre-OBD2, I assume there is no DTC for O2 sensor slow response, which could account for the peaks in the emissions.

What bothers me is that the peaks in the emission graphs coincide with acceleration, and so does the unusually loud and rough sounding engine. Sounds a bit like the old GM "Iron Duke" engine did on open throttle...

 

replace the o2 and have it ran again and see what it gets

 

Did just that. It passed with a new O2 sensor, so the car finally has new tags. Unfortunately they don't print the results out if the car passes, so I don't have the numbers to compare. Engine noise when accelerating is pretty much the same though.

 

intake manifold gasket could be bad, thats probably why its sounds like gm motor

 

I will check the vacuum pressure with a gauge (again), and maybe the FPR while I am at it. Then I am pretty much out of time. Too bad, b/c of the low mileage, but it is 16 years old. Thanks very much for the help.

 

anytime bro thats what we do best