I have been having an issue that is driving me nuts. I have replaced the following and all metered test appear fine but my car drives like it has a sock in the intake, it idles great, no check engine lights.
All new: fuel pump, fuel pressure regulator, cleaned and serviced injectors, new 02 sensors (working fine on diagnostics), new coolant temp sensor, new air temp sensor, tested mass air flow, new air filter, new Champion plugs, new coils...I ran some Chevron Techron through it and ever since (not sure if it's the causal factor or not) I've had this issue it seems.

What the hell? My car was running great, getting almost 21 MPG and then just hit the wall and I can't figure it out!

When I mash the pedal it goes but it almost feels like it's in a limp mode to prevent detonation or something. This MPG is killing me I feel like I'm driving a Chevy pickup truck.

Any input as to my "blind spots" would be greatly appreciated.

 

What are LTFTs and STFTs?

Are there any stored DTCs in the system?

Was the fuel filter replaced?

 

left field suggestion, but sticking brakes? Piston sticking in a calliper and dragging?

 

I'll get the fuel trims tomorrow. Fuel filter is also brand new, as are new plugs (again) and Amsoil 5/40 European 8.5 quart oil change as of last night.

It is not a dragging brake pad issue.

Thank you all. I will report back tomorrow with LTFT & STFT

 

You were careful not to get the O2 sensor plugs mixed up between upstream and downstream sensors? How's the thermostat, does it come up to temp?

 

Yes, I've had them all re-calibrated recently. 02's are not issue. The thermostat is brand new as well.

 

you mentioned a new fuel filter. It couldn't have been fitted the wrong way round, with the flow arrow pointing towards the tank rather than away from it?

 

Yes, the fuel filter is in correctly. There was no difference after the change.

 

I assume all the changes, tuning, and adjustments were accomplished in response to the problem, AFTER it was manifest, in an effort to cure it? If so, they haven't changed things, so i'd focus on the events prior to, leading up to its onset. You dumped a bottle of stuff in the fuel tank? Anything else out of the ordinary? Heavy rain? Standing water? New, never tried before fuel station?

 

No water, nothing like that. I am beginning to think my cats have clogged. The center section was replaced 7 or 8 years ago. Car only has 121K miles. Could the primaries fall apart and clog the ones under the seat. In the process of removing the down pipe and having a b!tch of a time with the rear bolt on bank 2. ARGH!

 

I am confused about the order of events here. On re reading your original post, it suggests you changed out the various parts etc, perhaps as part of some routine maintenance, and presumably without incident, then put the additive in the tank, after which you had the problem. Is this correct?
I am not familiar with the additive in question, but what is it supposed to do? Have you used it before? Have you used up the tank of gas with the additive in it, or has the problem prevented you from using the car? If there is an issue with the additive, you may need to revisit items you checked before putting it in.
You mention the car idles smoothly; does it start easily?
Sorry for the barrage of questions, but I am not sold on the cats being blocked theory just yet, especially with with the sudden onset of such a significant deterioration in performance.

 

Oh, and your new air filter couldn't have become contaminated with oil/water/additive?

 

A deep socket, swivel, and 2 extensions from underneath. Been there, done that!

It's certainly possible the cats are plugged. I doubt the downpipe cats would plug, as they are a metal substrate, similar to the wavy core of corrugated cardboard and coated with the catalyst material. The underfloor cats are ceramic I believe.

 

STFT Bank 1 is 3.1 and Bank 2 is 2.3

both LTFT are 5.5

Does this seem right?

 

Seems quite good to me, indicates that there is little correction from the mapped values in the ECU's table.

 

Hmm, what other parameters can I share to shed light on my issue?

 

We've covered everything obvious, except for exhaust plugging. Can you get a backpressure test done? Maybe the catalyst is plugged, or a baffle in the muffler has rusted and blocked the flow?

You mentioned disconnecting the exhaust, did you try driving it that way and see if the performance came back?

 

I knocked out the pre-cats int he down pipe, I can tell the engine breathes better but no difference in mileage still around 13.3 MPG.

How do I do a back pressure test?

Also, the MAX I can get my throttle open is roughly 72%. This has no bearing on the MPG but my throttle cable is totally at the end and almost increasing idle with the tension. Is this fixable or do I replace the throttle cable?

 

Never done one, but I believe a pressure gauge can be put in place of one of the O2 sensors ( the upstream one in the case of an X300) and then then measure how much pressure there is in the exhaust. Can indicate if it's blocked.

I suspect if any cats are a problem, it will be the underfloor ones, not the ones in the downpipe.

The cables are adjustable, there is a turnbuckle sort of arrangement on the cable. If you have traction control I believe that cable is also adjustable. It's been a while since I looked at them.

 

My cats have been replaced in the last few years and the car only has 123K miles, I guess it's possible? Maybe the mufflers are clogged?