: bad fuel economy, rough(er) idle, need help!

05-12-08, 12:47 PM
I need your help once again. :ill: My 2000 STS is still suffering from decreased fuel economy and an increasingly rough idle, even after a bunch of cleaning/fixing/replacing. I realize that my engine's cams are causing the bulk of the rough idle, but it's getting worse with time...

A bit of history, at 75,000 miles my car was running great, no complaints at all. I could feel a slight shudder at idle, but nothing to worry about. 65 MPH with no wind and mild temp and humidity would give me about 27 MPG on flat pavement. Dropping to 60 MPH would give me 29-31 MPG. 70 MPH was about 25 or 26 MPG... :D

At around 90,000 miles my idle started to become rougher, and around 100,000 miles my fuel economy started to drop. At close to 115,000 miles I followed the forum advice... cleaned the Throttle Body, replaced leaky plenum, replaced fuel injector o-rings, replaced intake gaskets, installed new OEM plugs and boots, new air filter. The only change I noticed was faster acceleration. :suspense:

I checked and rechecked the Fuel Pressure Regulator. I eventually replaced it with a brand new FPR which came with new o-rings preinstalled. I also changed the PCV and its grommet and verified that the new PCV was working properly. I tried cleaning the EGR valve, and when that didn't help I installed a brand new EGR valve and new EGR gasket. Still no improvement. :crying2:

Still no codes. The only codes I get are related to removing the battery. My 60 MPH fuel economy has been around 24-26 MPG for the last 15,000 miles. My 70 MPH is around 21-22 MPG. I will sometimes get 27 MPG at 65 MPH for a minute, then it drops back down to 23 again. :crying: The idle has gotten rougher to where my passengers sometimes ask if the engine is ok when we're stopped at a stoplight. I can now see the tach needle slightly wiggle at idle.

I'm at a loss, I'm not sure what more I can check or replace to try to get my fuel economy back to where it was last year. I have tried different types of gasoline too. My tires are newer and are very similar to what I had before. I didn't notice a MPG difference after the tire change. The only things left I can think of would be fuel filter, fuel pump, injectors, and coil modules.

Any ideas? This is driving me crazy. The car still has tons of power and acceleration, probably even more now after all of this work. I realize the idle will never be perfect on this 2000, but I would like it to idle like it did 25,000 miles ago. And I would like the fuel economy I had 15,000 miles ago.

Thank you in advance! I am sorry for pestering so much about these issues, but I love this car so much and I just want to conserve some gas and have it run like it did just last year. Plus I can't help but seek advice from you gurus! :D

05-12-08, 01:32 PM
In order to completely or partially rule out injector(s), dump a 20 oz. jug of TECHRON on top of a half tank of 93 and go for a highway drive. BUT, if you were getting a misfire (plugs, coil, injector, etc.) you should get some sort of 0300 code set, and a system code as well.

05-12-08, 02:30 PM
no codes at all? how are you checking your codes?

05-12-08, 02:32 PM
have you looked into the problem possibly being the crankshaft position sensors? They can cause a rough idle, but at high rpms, not too noticeable.

05-12-08, 08:32 PM
I am checking my codes from the dash info screen and buttons. I hold down ON/OFF and INFO ^ until the dash resets and offers to show me ALL CODES. The display cycles through a bunch of categories, mostly saying things like "NO PCM CODES". For the past year, I have only seen codes releating to low battery and my removal/replacement of the driver-side window button module.

Both crank sensors were replaced about 25,000 or 30,000 miles ago. The engine would die after idling for a few minutes. One sensor threw a code so my stealer replaced them when I had my car for an oil change and brake job.

I will do the Techron thing soon. I live in the boonies, so I have to drive to another city to get 93 and Technron. In the mean time, I tried 91 and JB Injector Cleaner last week, but didn't notice a difference.

05-12-08, 08:37 PM
How is a misfire detected?

I ask because sometimes this rough idle almost feels like it is missing. I'm no expect and I'm no mechanic, but some of the more severe shaking and wiggling idle moments feels sort of like working on an old 4 cylinder engine that's only hitting on 3 cylinders.

But...my exhaust smells normal and the car has plenty of power. It just feels sort of restless and whacko when I coast to a stop and am sitting there at the stop.

And.... of course, the 4 or 5 mile per gallon reduction in fuel economy over the past 15,000 miles. Sometimes I get better fuel economy for a minute or so, but then it drops back down just again. :ill:

05-12-08, 08:57 PM
A slight misfire will not set a code. When my '97 started it after having the fuel rail replaced, I cot no DTC until I brake torqued it and induced a sever enough misfire for a few seconds. THEN I knew what the problem was. Did you use A/C Delco plugs? Also check the connection and grounds on the coil cassettes.

05-12-08, 09:00 PM
Yeah that ^^^ and back to your throttlebody work.....there's also a set of bleeds and an Idle Speed Control. Looks as if you've covered the big bases, and the car appears to run OK. The idle is screwy.......but the overall fuel economy is a mystery. (Use the long term fuel averaging, over several tanks of gas. The instantaneous is so quick and sensitive that it will show a glitch if you cough....) Can the TECHRON idea if you've already tried a slug of system cleaner.

Take a good look at the air intake piping from the air cleaner side of the MAF to the throttlebody. Clamps tight ? MAF and adapter look properly oriented ? A small air leak between the MAF unit and the TB can cause mayhem with low speed fuel trim. The calibration on the MAF is unbelievably tight: it hates any sort of air leak.

Let me sleep on this one..........

05-12-08, 09:01 PM
Yep, I used the exact same Delco/Denso plugs, I don't remember the exact model number offhand. New boots too. I thought the ground spring looked fine. I can try removing, cleaning, and replacing the coil modules and see if that helps at all. If they weren't so expensive, I'd just replace them.

05-12-08, 09:05 PM
I'll double check the MAF and piping.

For what it's worth, I gave the MAF a blast of CRC MAF cleaner spray following the instructions (kept the nozzle about 8 inches away, very short blast to wet the MAF) hoping that would help. But again it did nothing.

Always used Delco or FRAM paper filters. This car has never seen any K&N or similar oil filters.

05-12-08, 09:06 PM
The chances of a coil pack going to lunch are awfully slim........check the multi-pin connectors, though.

05-12-08, 09:15 PM
If you remember from your cleaning exercise......does your MAF look like the RH (OEM) unit with the honeycomb flow straightener ?? See the three tiny heated resistors between the bridges ??? They should be clean and bright, and they're fragile as h..... the little black teardrop is the Intake Air temp sensor. The whole enchilada is calibrated, so don't get creative.

Click and scroll in.......BTW, the left unit is obviously a JET modified stock unit....modified mechanically, not electronically.......caveat emptor.

05-12-08, 10:49 PM
It has the honeycomb. The resistors are not shiny but they're not caked with dirt either. I sprayed the whole works for a couple seconds with CRC MAF cleaner spray just to see if it would help.... it didn't change anything.

The honeycomb and resistors have sort of a thin chalky, milky dry haze... so does the inside of the tubing. Does that mean anything to you?

05-13-08, 08:37 AM
That haze is nothing more than a very thin, very dry coat of microscopic dust which eventually builds up in the intakes of anything that uses a lot of air. Your finger would probably rub it off or hide it with body oils.

Now I'm out of ideas................but will keep thinking.......

05-13-08, 02:36 PM
Could my power steering pump be dragging the engine down at idle? The PS pump is louder than it used to be, but it's not leaking and it's not making that "dying power steering pump" sound. Power steering has a few hiccups once in awhile, sort of a hesitation or shudder during turns, but not enough to concern me too much... but it is getting louder, sort of a metal on metal mechanical sort of sound.

05-13-08, 06:53 PM
I noticed today that when I come to a stop, my tach drops to about 800 RPM for a few seconds, then drops down to about 600 RPM. Is this normal? To drop to one idle speed, and then drop again a few second later?
Also, I should mention that after I cleaned the TB, all was well for the first few minutes I drove the car. But at one stoplight, the "idle" reved to 2200 RPM! Needless to say, there was a heck of a jerk when the transmission engaged again! This happened a few times before I opened the hood and noticed that the Throttle Position Sensor cable seemed to be loose. I unplugged and replugged the cable (I think it's TPS, it's on the front (bumper side) of the TB) and the reving went away.

05-14-08, 05:02 PM
I'm going to send my injectors in to be tested and cleaned. Probably overkill, but everything else checks out for now, so I'm out of options.

As an update... today's weather was perfect.... 65F, 35% humidity, no wind at all. So I took my car out on a flat highway and ran it WOT for a few runs, letting it rev high for a few seconds between shifts. I set the cruise control at 65MPH and watched the INST and AVG fuel economy. For the first mile, it was a solid 27 MPG (26.7 avg). Then, like usual, after a slight dip in the road, it dropped down to 24 MPG (24.4 avg) and stayed there for 4 miles, until I stopped and turned around.

On the return trip, the exact same thing happened. After WOT acceleration, I was getting 30 MPG (29.6 avg) when I set the cruise to 60 MPH. After a couple miles I hit a slight dip in the road, and the economy dropped down to 26 MPH (26.1 avg) and stayed that low for the rest of the drive back to town.

I was so spoiled by a constant 27 MPG at 65 MPH.....