Last fall when I replaced my exhaust system from the CAT back I ran a straight pipe where the first muffler normally would go.

Any idea if this is why my MPG sucks now?

Thanks,

Matt

What engine?

What fuel injection? LH 2.2 or 2.4 ? (maybe LH 2.$ ....?)

Michael

89's are b230f's with LH 2.4

Yeah, what jwalker said.

Start with a tune-up. Had some bad u-joints killing my MPG awhile back. They were groaning as I came to a stop almost like it was going to stall. They were seized all the grease nipples were busted, car only had 150K on it too.

Tell me where to look and I'll check it out.

Thanks,

Matt

Also check the plugs to see if the engine is running rich.

The back pressure change may have some thing to do w it,,,but I would check 2 see if the brakes are dragging...

The rear rotors and pads were replaced just last week as one was stuck, but I'm still experiencing bad MPG.

Did the MPG go down from the first week you did the exhaust?


My first thought of bad MPG is a failing Fuel pressure regulator.
Pull off that front hose on the regulator and look for wetness (gas). it should be dry
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Yes it did, but the brakes also started about that time to. Thought it was front wheel bearings all the while. Did not realize it was the brakes until I heard that grinding noise. Ouch!

Unlikely low MPG has anything to do with your exhaust mod. When I changed over to the IPD 'sport' exhaust over 10 years ago it made zero difference in fuel mileage.

Any other symptoms? Change in driving habits or routes (e.g., city vs. highway)? Greater use of AC? If you have the AW70, is it kicking into OD as it should? Does the engine still idle well? Air filter in good shape? Checked for vacuum leaks? Oxygen sensor OK?

Smitty,

You bring up some good points, so here goes.

The car has no check engine light on and it appears to be running fine. The idle is just under 1000 RPMs. I will check tonight to make sure no codes are set just to make sure.

The car has 320,000 miles on it as of this weekend. It is a 1989, with an AW70 transmission in it. I've used the A/C recently, but this has been going on since the fall and I thought I had fixed it by replacing the one stuck caliper a few weeks back. Unfortunately that did not fix the problem.

I know I'm rambling here, but I'm trying to get in all my points here.

Normally when I've driven the car in the past and now, I'm able to get between 25 and 27 mpg with a combination of around town and highway miles. Mostly highway.

The air filter is new as that was replace last fall as well. I have not checked for vacuum leaks, but will now. I also just replaced the connection between the MAF and the throttle body last fall as that had a small crack in it bellows that was very had to find.

As far as the transmission kicking into OD at the appropriate time, that I'm not sure of. I'm reading right now about how to adjust that as it seems to me that it takes to long to kickdown particularly when I'm using cruise control on hills. As with other cars that I own, when I hit a hill with the cruise control on, the transmission normally kicks down at the slightest change in mph, where as the brick does not. It usually will drop off a good 5 to 10 mph before kicking down on its own, but it will do it right away if I mash the peddle to the floor.

Finally, the O2 sensor as far as I can tell has never been replaced and I've owned the car since 1996 (187,000) and have never done anything with it. Is there a reading I can take to find out if it is out of spec? If it is then that is a possibility that I'm running in open loop mode and never getting into a closed loop mode.

Thanks in advance,

Matt

Well there ya go. Do a complete stage O which includes a new O2 sensor. I wouldn't trust a stock Bosch sensor to deliver consistent voltage sweeps after 60-70 k miles.
And the most obvious reason why the car gets bad mpg after the cat-back exhaust was put in: ALL exhaust shop mechanics like to twist up the O2 sensor wires when taking them out to re-install in a new pipe/cat. BTDT. You should undo the harness and spin the cable so the wires don't kink or bunch up. Any scrapes, kinks or breaks in the wires will make the O2 sensor external air reference not be there.

ok, I'll look at the O2 sensor, but it will have to wait. The transmission (AW70) died yesterday and now all I have is 1st gear and reverse, nothing more. I guess with 320,000 miles on it it had finally had enough. Going tomorrow to find a later model M47 to swap into it. Always wanted a 5-speed in this car, but this is not the way I wanted to go about it.

Thanks,

Matt

Hi,

Have you flushed the trans fluid lately? Maybe it just needs flushed. I would exhaust possibilities before finding another trans.

On the O2 sensor, the orig. Bosch 13034 can be found on Ebay for $64 w/ free ship.

I was going to flush it this weekend as it was time for it to occur. I was also going to drop the pan and clean the screen and what ever had collected at the magnet, but then I have to spend about $70 bucks or so on fluid that may not work.

So today I went to the PNP and found 2 M47 transmission's with cruise control. Both are 1990 models so I believe they are more robust than the earlier models or am I wrong there.

Anyways, Saturday is D-day for removal. Can't wait.

Well, a couple of things about vacuum leaks. Big ones (like it sounds you experienced with the inlet snorkel) will cause the idle to deteriorate badly. Small ones are less noticeable but typically result in poor off-idle performance; they aren't noticed as much at cruise speeds due to a wider throttle opening. So do give all your vac lines a good look-see, and don't forget the one to the brake booster AND your cruise control.

To check the O2 sensor you need a digital voltmeter, or even better, a scope. There's lots of info available on the web, but essentially you're looking for the sensor output to oscillate between certain high and low limits. Here's one link to get you started: http://www.ngk.com/learning-center/article/151/how-do-i-test-an-oxygen-sensors-performance-t

Just make sure your car is warmed up before you start testing the O2 sensor. You should find the connector at the top of the firewall on the passenger side - easily traced from the sensor itself.

If it turns out you need a new one, you can save about $50 by getting the same Bosch sensor for a '92 V8 Mustang; should be PN 13942 at Autozone. PN13953 is the same but with a longer wire. You may need to splice your old connector onto the new sensor, but that's worth $50 in my book.

Lastly, have you looked at your spark plugs to see if there's evidence of running rich? Given your car's age, the fuel pressure regulator could be getting tired or sprang a leak. In fact, when you check your vacuum lines, sniff the one going to the FPR to see if it smells like raw gas. A really bad FPR failure will make the car stop running quickly, but small leaks can happen (although they're rare).

Just an added note on the O2 sensor testing:
If the voltage sweeps appear to be consistent using the correct diagnostic tool (say Hurray for OBDII diagnostic tools to read the O2 feedback real time on cars equipped; can't do easilly or w/o an oscilliscope on the LH x.x cars) the difference in amplitude of a old 'weak' sensor sweep compared to a new sensor will not be obvious to the untrained technician. You certainly cannot tell the condition of the signal sweep on a DVM.
O2 sensors get 'lazy' and the timing response as well as amplitude of the signal get weaker with age, until they essentially flatline and give a more or less constant low voltage output. When the ECU sees this type of signal it is read as if the car is running leaner than it should and the ECU will enrichen the mixture. The ck engine light won't light until the sensor is nearly completely toasted, so you could be running on the rich side of tune for years and not know it.
When in doubt swap it out. O2 sensors are like spark plugs, 60k and toss em even if you think they still work. It is far easier to not guess (or buy diagnostic equipment)and be sure it gives a good signal.

Just an added note on the O2 sensor testing:
If the voltage sweeps appear to be consistent using the correct diagnostic tool (say Hurray for OBDII diagnostic tools to read the O2 feedback real time on cars equipped; can't do easilly or w/o an oscilliscope on the LH x.x cars) the difference in amplitude of a old 'weak' sensor sweep compared to a new sensor will not be obvious to the untrained technician. You certainly cannot tell the condition of the signal sweep on a DVM.
O2 sensors get 'lazy' and the timing response as well as amplitude of the signal get weaker with age, until they essentially flatline and give a more or less constant low voltage output. When the ECU sees this type of signal it is read as if the car is running leaner than it should and the ECU will enrichen the mixture. The ck engine light won't light until the sensor is nearly completely toasted, so you could be running on the rich side of tune for years and not know it.
When in doubt swap it out. O2 sensors are like spark plugs, 60k and toss em even if you think they still work. It is far easier to not guess (or buy diagnostic equipment)and be sure it gives a good signal.

Tick-over for autos is usually 800rpm (manuals 900). I think, and I need expert guidance here, that if tick-over is set high with F.I. it gives negative affects with regard to mpg right across the rev range. Setting the tick-over slow also gives much less auto creep and into drive clonk and makes town driving more pleasant. Not so much holding on the brakes needed.

No way that would cause a reduction in MPG.Something else is amiss with your car.