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  #1  
Old 04-15-2011, 01:16 AM
00M6SSLS1 00M6SSLS1 is offline
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Posts: 10
Default Rough Idle, Stall and Lack of Power

Hello Everyone,

I'm new here as you can tell but I hope to stick around and help out.

My dad picked up a used 1999 Mazda B2500 which has a 2.5L inline,
4 cylinder (8 spark plug design).

The engine idles rough, and stalls if the throttle is blipped and allowed
to return to idle. The engine in general lacks power overall.

We started by checking some vitals, and then replaced all eight spark plugs,
wires, PCV valve and all fuel injectors since the intake was off anyway.

Here are the measurements and info:
Fuel pressure at idle 70 PSI

Compression Cylinders 1-4:
198, 202, 200, 210 PSI

No DTC codes present

Plug appearance seems fine on all cylinders

02 Sensor #1 swings from 110 mV to 880 mV
02 Sensor #2 swings from 100 mV to 800 mV, and then flat lines once warm

Short term fuel trims are +10%
Long term fuel trim is 0.8%

My next step is to check the EGR valve, vacuum lines for cracks/loose fitting, and the IAC valve.
They all looked fine, but I'll have to double check everything as the tune-up improved nothing.

If anyone is aware of a notorious issue with this motor, please post
some tips and info as we're running out of things to check.

In the mean time, I'll search more threads for answers. Still nothing
jumping out.

Thank you!

Last edited by 00M6SSLS1; 04-15-2011 at 02:39 AM.
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  #2  
Old 05-11-2011, 11:20 AM
VegasRanger02 VegasRanger02 is offline
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Posts: 416
Default Re: Rough Idle, Stall and Lack of Power

Check the Idle Air Control Valve, that helped me a bit, I ended up replacing it. Try cleaning it with throttle body/ carb cleaner. Also, maybe you need a new camshaft syncro. They can cause wierd problems. Before I changed mine, I couldnt go over 70 with out my truck hesitating and jumping like it was gonna blow up.. wierd.
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  #3  
Old 05-12-2011, 02:07 AM
00M6SSLS1 00M6SSLS1 is offline
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Default Re: Rough Idle, Stall and Lack of Power

Sorry for not updating this sooner. It appears the engine has an, "internal vacuum" leak.

After a week of messing around with the motor, and everything
checking out fine, I gave up and sent it away to another mechanic.

He states that air is passing by the valve guides and causing the
poor idle. I'm not sure how the compression readings were so high.
I even did a leak down test and found nothing.

At this point, we're likely going to return the truck and get into
something else. Thanks for the help in any case!
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  #4  
Old 05-12-2011, 04:28 PM
ddunfee ddunfee is offline
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Posts: 169
Default Re: Rough Idle, Stall and Lack of Power

Bad valve guides won't affect compression because the valve is closed and isolating the guides from the compression meter. Usually bad valve guides will give you smoke when you back off on the throttle and vacuum is high.
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Old 05-12-2011, 05:45 PM
00M6SSLS1 00M6SSLS1 is offline
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Default Re: Rough Idle, Stall and Lack of Power

My thoughts exactly. The plugs showed no signs of oil deposits,
and the exhaust smoke colour was normal.

If anything I'm thinking one of the intake valve springs is weak/broken
and causing the valve to bounce which may not appear during a
compression test if the valve seats properly.

We'll see, I may just pull the valve cover to check the springs. At
this point, nobody can pin it down.
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  #6  
Old 05-12-2011, 09:37 PM
knightmare1015 knightmare1015 is offline
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Default Re: Rough Idle, Stall and Lack of Power

I'd go ahead and do that. I also noticed something about your compression ratios too. The #1 and #3 seem to be a bit lower than the other 2 cylinders. You may have to have the heads redone if it's a valve problem. If cylinder head work is required, what do you plan on doing to the heads?
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Old 05-12-2011, 09:52 PM
00M6SSLS1 00M6SSLS1 is offline
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Default Re: Rough Idle, Stall and Lack of Power

Actually, I thought the compression readings were exceptional.
Aside from cylinder #4, the numbers are tight...and even #4 is
well within spec.

If the springs are broken/worn, I'll just pressurize the cylinder
and swap springs.

I don't plan on removing the head, and if it gets to that point I'd
rather just buy a used engine and change it out.
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  #8  
Old 05-12-2011, 10:06 PM
knightmare1015 knightmare1015 is offline
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Default Re: Rough Idle, Stall and Lack of Power

Quote:
Originally Posted by 00M6SSLS1 View Post
Actually, I thought the compression readings were exceptional.
Aside from cylinder #4, the numbers are tight...and even #4 is
well within spec.

If the springs are broken/worn, I'll just pressurize the cylinder
and swap springs.

I don't plan on removing the head, and if it gets to that point I'd
rather just buy a used engine and change it out.
I didn't say they was bad just a little bit off, but they're still well within specs. Kool idea about swapping the springs. I don't blame you a bit about changing the head. I've seen them cost as much as a good used engine myself. Some of the cylinder heads I worked with in the past, would cost as much as a house.
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