Hey guys,

I was hoping someone could help me troubleshoot an odd issue that I am having with a 2000 Mustang GT. It hesitates off idle. Also it has no top end power. It struggles past 45 mph. After a long drive, the front brakes are pretty hot. So I took them apart and there is no uneven wear and they seem to go in and out OK. If you press the brake and then let off, the wheel turns freely. The SES doesn't light up. I am gonna check the plugs in a while. The exhaust sounds normal, but I haven't checked for a clogged cat yet.

Any suggestions?


* The hesitation feels like the car is slightly power-braking before it "let's go" and the car goes forward.

 

One way to check for a clogged cat is to connect a vacuum gauge at the manifold. Start the car and watch vacuum. should be around 17 or so, i think. then gradually raise RPM to about 3000. Watch the gauge while holding steady at 3000. If vacuum drops significantly (not sure how much) while maintaining RPM, this would indicate exhaust restriction. If the vacuum drops slowly or quickly while idling, this indicates a severe exhaust restriction.

Another way is to remove the upstream o2 sensor and take a reading of pressure (if you have a good pressure gauge) while engine is idling. Should be around 3psi of backpressure (anyone else to confirm what the back pressure should be???). Do this on both upstream 02 sensors.

If you don't have a backpressure gauge, you can get one for about $40 online, just google "exhaust backpressure tester".

Not sure why the brakes are hot. That may be normal, seems like they function okay since you say the tire spins freely.

If you have access to a good scanner that can read sensors, get the MAP reading verses TPS, RPM, vehicle speed. May as well capture the O2 voltages and coolant temp too. That would really help to figure out your problem. You don't need a check engine light or trouble codes to show to get the sensor data.

I had a car under warranty that had a clogged cat. The dealer replaced the cylinder head, the fuel pump, the fuel rail, the computer, coolant sensor, all injectors, intake, and some other stuff. Also tried to tell me my oil was too thick (I went apesh*t when he told me that, anyway they changed the oil, not me). After being towed several times they finally found the cat was clogged. Go figure. Had hesitation and no power above about 30mph, then would start crawling and could not make it up hills. Yet another reason I don't like taking my cars anywhere. There was over $5000 dollars of warranty work done on a 3 month old $10000 car.

Before you take it anywhere and get screwed for the diagnostics, I recommend getting a scanner that reads the sensors. I have the actron cp9185 ($185). I love it (no, I don't work for them). I can also save the data to my pc so I can compare later. Considering a diagnostic costs $100 and up, its well worth the price. Codes only tell you so much, seeing the raw data makes a huge difference.

Good luck.

 

Wow, great advice. I don't have the scanner or a vacuum gauge. I was just going to drop the H-pipe and visually inspect it. I also bought some new plugs, so I'll check them as well.

 

One thing you could try is to actually loosen the h pipe and take it on a test ride. It'll smell and be noisy, but the reduced back pressure should resolve whether or not the cats are stuffed.
Don't run it too long with the pipe removed, as supposedly exhaust valves can get burned with the pipe removed (I've never seen this happen, but I was told about it years and years ago)... Maybe thats just a myth... I don't know, never tried.

 

Thanks for the tips!

I did a brake job and they seem fine. Afterwards, it ran better. It had a lot more power. I went for another ride in it today and it sounded kinda bub-bub-bub-bub like the Jetsons. Which sets me in the direction of clogged cat. It had more power this time, but then it threw a code P0308. I'm gonna check the plugs next. I bought some new ones anyway, because I doubt this thing has had a tune up since it was new. The P0308 lists these as possible causes:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer


After the plugs, I think I'll drop the H-pipe and check that and then do the fuel filters.

 

Check for vacuum leaks, too. P308 is simply a misfire on cyl 8. I just fixed a car throwing these codes and found the injector seals were bad. Fixed my wifes car throwing those codes 2 weeks ago, was a bad 02 sensor. Both threw the P30x codes.

 

OK, will do, thanks.

 

After the tune up and fuel filter, the car still has what sounds like a slight miss. It still sounds like "bub-bub-bub-bub" and is more pronounced and it still has little power upstairs. No codes, no ses light. I think I'm going to try a new fuel pump next, unless anyone has any other easy troubleshooting tips?

 

OK, I think this is good...

A few days ago I took the car to get a little gas. The low fuel indicator light came on, so I went and put $5 in so that I don't run out, but not so much as to make it difficult for when I do the fuel pump. It "missed' the whole time (to and from the gas station) and ran as posted in my last post.

I just took the car for a ride to heat up the exhaust manifolds (to test for heat variances) and I turned on the A/C and drove about 4-5 miles. The first two miles the car ran perfect. No bub-bub-bub and it had plenty of power. Then I turned off the A/C and after a short while the "slight miss" came back and it had less power. So I then thought that it had something to do with when I turned the A/C off, so I turned it back on for the last mile and it made no difference.

At least if it's intermittent, it probably isn't bad compression, right?

Still no codes...

Anyone have any more suggestions?

 

I am pretty certain that the computer is notified when the A/C is running, and it steps up the idle speed to compensate for the additional load. I don't know if it does anything else while A/C is running or not.

Possible things to check for would be leaking EGR valve (not closing all the way), O2 sensors, etc.

I doubt the cat converter is an issue, since you say the problem went away with AC on. If the cat were clogged, you'd see the problem with or without AC. Sounds more like a vacuum leak (very slight) or a fuel system issue, both of which can cause lean, misses, rough idle, hesitation, rich, poor fuel mileage.

Oh yeah, if its still missing, you'll get your codes back. Also, heat variances will just confirm what you already can get from the codes, and that is which cylinders are missing. You're better off getting a scanner that shows the actual running voltages of the sensors if you can borrow one...