I was having a rough idle problem. Cats were clogged and injectors clogged. I was getting 17 miles per gallon 20 plus on highway. Put true dual exhaust from exhaust manifold to tips, replaced the injectors, MAP sensor, new plugs, ignition module, Plug wires, new distributor cap, rotor, brake booster, fuel regulator, fuel pump,fuel filter and all the vacuum lines. I am getting good fuel pressure and vacuum tested good. The car runs great (strong) but the mileage is averaging 14 miles per gallon.
Does anyone have any test that I could try?
Thanks

 

How's the idle now? Be very objective to answer. It will be tough to say it's still rough, after your new parts offensive...


Any black exhaust at idle?

 

according to my meter it is running about 780 RPM. But about every 30 seconds you can hear the idle jump up a little bit over 900 RPM just for about a second. I cleaned out my exhaust tips this weekend and still no black carbon (driven about 300 miles).

 

Is emissions testing required where you are for vehicle inspection? That would go a long way toward figuring this out. Idle and cruise speeds would indicate the condition of the engine's fuel burn.

-John

 

I would say vacuum leak, but you're saying every 30 seconds you get a jump for 1 second. That is an electrical problem; not mechanical.

If this RPM fluctuation is NOT regular as you describe, then you have a vacuum leak. That will make an irregular jump in rev's.

You should time this. It will make a proper diag much easier, unless you have a code set. Are you sure the code light lights up? Does the SES light come on when you turn on the key, BEFORE YOU START THE MOTOR?

Anybody else?

 

Also, O2 readings would go a long way towards diagnosis. Sounds like car is lean if you have no black smoke from exhaust. Had similar problem with my 86. Company had sold me 305 injectors, 19lb, instead of 350 injectors, 22lb, and it idled like crap and hesitated (too lean). Double check ignition timing if your car is an L98. If it is, did you disconnect "Set Timing" connector to put it in base timing before setting timing? Was your MAF a rebuild? If it was, chances are it was an A-1 Cardone brand and they are crap. That will cause lots of driveability problems. Check you vacuum supply to your MAP sensor. Did you remember to plug the MAT sensor back in under the intake plenum (if it is an L98)?

 

I don't understand this (along with a few other things too, but that's another story).

Seems to me that all that can be told by O2 readings is that they're out of normal parameters, because of something else that is already wrong, and making for a faulty air/fuel mix to start with...

???

 

Rich/lean diagnosis, how quick the O2 is switching (cross counts). Could be a MAF, ground, MAT, so forth. Watching an O2, with enough experience is almost as good as a gas analyzer and I have watched a bunch of them over 25 years.