94chevyrocket

I have a 94 z28,LT1.Hers's my situation.I recently changed the following parts;fuel pressure regulator,fuelpump&filter-distributor,coil-iat,ect sensors,and the PCM.I cleaned the throttle body and MAF sensor.The engine was running a little bit rough,at times I smelled fuel,and I was getting around 10 mpg.One month later I changed the spark plugs(NGK)-turns out plugs were loose...Funny thing-after changing the plugs(gapped at .50) my idle is a little high,and when I'm on the highway at about 60 mph the service engine soon light comes on.When I exit the highway,after a couple of minutes of street driving(30 mph or less)the light goes off.No improvement in gas mileage since changing the plugs,noticeable improvement when accelerating........I figure since the car has 120,000+ miles it won't hurt to change the throttle position sensor,and idle air control valve,and o2 sensors.Any other input would be greatly appreciated...thanks

RamAir95TA

Stop wasting money by throwing parts at the car. Have the PCM scanned. The SES light is there to tell you a specific problem. No use speculating. It will give you a good place to start.

94chevyrocket

Well....I had the pcm scanned,that's what lead me to change the distributor/coil(shut off problem prior)-the fuel regulator broke,fuel pump froze(that's why I changed those).The other parts were cheap so I tried them(prior to the scan).one BIG problem-94=OBD1,no place seems to have the scanner for that,I did find one place though and the reading is very vague,it's not specific like the OBDII.So I was hoping to get lucky here,meaning maybe someone had the same problem and found the solution.I heard there's some new software/program that w/ a laptop plugged to the pcm is more specific than the original OBDI scanner-anybody know about this??

RamAir95TA

Pick up Datamaster (TTS Power Systems). It's free and is invaluable. Also pick up a data cable and interface for your computer/laptop. Nowadays stuff like that is priceless when dealing with an 18 year old fuel injected computer-controlled V8.

You need to do a real-time scan of the PCM to see what it's doing while the engine is running improperly. Getting the correct codes pulled is also important. I would start here first.

When you compare OBDII to OBDI the codes are much more vague but they are no less informative than OBDII.

shoebox

No problem in replacing things that are broken, but don't waste money replacing things that are not. Sometimes new parts are as not as good as the old, working parts and you just add trouble to ones you already had. It is a good idea to pursue getting a scanner or software and cable to diagnose your car. Either is a good solution. You can find many solutions for scanning with a search in the computer/diagnostics forum.

Your scenario sounds like an EGR issue in that it occurs during cruising, but goes away later. However a scan will tell you for sure. You can find a list of f-body specific trouble codes in the link in my sig.

94chevyrocket

Thank you for the replies,I will follow up on the advice and keep you guys posted.

Alexag562

I had the same problem I did everything I could replace the catalytic converter, the tps, clean out the fuel injector, tried sea foam, and nothing. Till I ran into a post that suggested my egr valve and guess what I took it off and took it apart and cleaned it with carb cleaner and boom that did the trick fix my service engine light problem gave the car a way better feel and felt it a bit faster.