Rough idle

Tiny
MNDANDREWS
  • MEMBER
  • 3 POSTS
Cool!

She is bringing the car home this weekend from school, I'll have her stop by the mechanic and have him check out the compression and injector on that cylinder.

Thank you so much for the help. I will let you know how it goes after Friday's visit! ;-)
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
BEAU123
  • MEMBER
  • 4 POSTS
Thank you for suggestions - now I feel a bit more comfortable that it might be the Idle Control Valve.

I'll keep you updated with the results.

And I'll definitely look into the position sensor. Thanks Again!
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
RANDY HARPER
  • MEMBER
  • 2 POSTS
  • 1998 MAZDA 626
  • 2.5L
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 135,000 MILES
Jumped time. Put new timing kit in and now engine idles rough, it is kind of surging when in gear. I took timing cover back off checked time again it is good. Why is it running rough?
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
LEGITIMATE007
  • MECHANIC
  • 5,121 POSTS
Awesome
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
CARADIODOC
  • MECHANIC
  • 33,916 POSTS
The 2.5L is listed as an "interference" engine, so it likely has bent valves. The best test to identify that is a cylinder leakage test. If valves are bent, you will hear air hissing from the tail pipe and/or throttle body.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
RANDY HARPER
  • MEMBER
  • 2 POSTS
If so why? It has plenty of power when driving with no miss just at idle.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
CARADIODOC
  • MECHANIC
  • 33,916 POSTS
Many years ago I had a Dodge Dart with a 340 c.I. That I raced. It had a badly burned valve that I later found to be about one-fifth missing, (burned away). It did sound like an air compressor at idle, but it had all kinds of power. You can have valves that are leaking far less than mine was, but you have to remember, my idle speed was controlled with an adjustable screw. Yours is controlled with a computer that is expecting to see a steady vacuum reading at idle. Leaking valves will cause a reduction in vacuum and idle speed, then the computer will try to correct the idle speed while including that unknown variable in the fuel metering calculations. The electronic controls and the mechanical characteristics of the engine are fighting each other instead of working together. Most of the time the engine will not start or run with bent valves, but that depends on how much the timing belt jumped. You do not have to worry when it jumps one or two teeth, but there are some engines that cannot tolerate it jumping three teeth. If your timing belt broke while the engine was running, it is almost a certainty there are bent valves. (I have to add that my source has been wrong on at least one occasion. They list the 3.0L in my 1988 Grand Caravan as an interference engine, and I know it is not).

Sometimes you can get an idea of the condition of the valves by turning the crankshaft by hand, with a ratchet. It will start to turn hard when a piston comes up on top dead center on the compression stroke. If you know what "normal" feels like, you may recognize when it turns too easily. A helper may hear hissing at the tail pipe. You should never hear that at the throttle body. During the exhaust stroke when the exhaust valve is open, you can't turn the crankshaft fast enough by hand to make the normal hiss.

Since you are confident the belt timing is okay, I would do the cylinder leakage test on at least one cylinder. Also, consider that it is not uncommon on some engines for the dowel pin, (when that's what they use), to shear off the camshaft sprocket, then the sprocket turns on the camshaft. That makes cam timing late even though the sprocket is right on the money.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
LAURENTMOLTENI
  • MEMBER
  • 3 POSTS
  • 1998 MAZDA 626
Engine Performance problem
1998 Mazda 626 4 cyl Two Wheel Drive Automatic

I have a Mazda 626 1998 4cyl with 106000 miles on it! Idling is rough as soon as I put a load on it. If I put the AC on, it stall! I changed the fuel filter, air filter, spark plugs, spark plugs wired, checked the EGR, cleaned both the Idle Air Control and Mass Air Flow sensor. The IAC was a little black but I do not see any improvement after cleaning it! Is it because you are supposed to reset the computer and if yes how do you do it? What else could be wrong? Please help I need AC (in Tucson AZ).
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Inspect and test the throttle position sensor's closed and wide open throttle voltages, also check the PCV valve and also check if it has an idle up solenoid might not be increasing the rpm when AC clutch engages the compressor
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
LAURENTMOLTENI
  • MEMBER
  • 3 POSTS
Thanks Rasmataz. Any trick to test throttle position sensor manualy? How do you check the PCV Valve because when I pull it it has suction?
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Back probe the TPS reference voltage should give 5volts and also the return signal wire should gradually increase voltage to that amount in a linear way with no breaks -

PCV valve -shake it should rattle if not clean it
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
TAM
  • MEMBER
  • 2 POSTS
  • 1998 MAZDA 626
I have a 1998 Mazda 626 with 124,000 miles that purred like a kitten until 2 months ago. Since then the car has idles rough and the check engine light has come on several times. I had the 02 sensor fixed, the MAF, the air filter changed, the plugs and expensive wires. I have used STP and fuel injection cleasner and nothing has worked. I have service comments from a shop which state: No Codes! Will Start With Intake-Gasket. Please advise if this could possibly be my problem. The price is &288.00. I cannot afford to fix anything else but the problem.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
BUMPYSBRO
  • MEMBER
  • 10 POSTS
Does the idle issue happen before or after the car warms up? Or is it all the time? Could be a vacuum leak or a temperature sensor malfunction.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
LAURENTMOLTENI
  • MEMBER
  • 3 POSTS
Thank you so much I'll try that. Really appreciate your feedback.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
BEAU123
  • MEMBER
  • 4 POSTS
I'm having the same issues with a 98 626lx at appx the same mileage. Check engine light had come on with an Idle Control Valve error code.

Where you able to fix the issue? If so, was it a vacuum leak?
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
BLKLKI
  • MEMBER
  • 1 POST
  • 1998 MAZDA 626
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 170,000 MILES
Ive recently changed the transmission in my 1998 Mazda 626 and now i'm having cooling problems. Ive changed the thermostat and replaced one of the fan motors on a radiator fan (it was burned out). I have a small leak in the radiator on the side of the engine where the air intake is. The car will run at normal temperature for 20-30 minutes before it starts over-heating then shoot right up to the red. The water levels have been anywheres from normal to way low when the car overheats. Also, the car has started to idle real low, at about 500 rpms and the car wants to shut off when standing in idle. Any solutions or opinions of what all may need to be changed is much apreciated.
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
YAM
  • MEMBER
  • 23 POSTS
Replace the radiator! Would be the first thing I would do, if it has a leak, then you may see the car acting funny! I remember this happened to me when I had a chevy nova 1987 model (love the car which was basically a toyota corrolla) but the radiator had a small leak/crack, had to change the whole thing out and it worked fine afterwards! That may be a good start to it!
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Sunday, January 5th, 2020 AT 6:50 PM (Merged)
Tiny
2CP-ARCHIVES
  • MEMBER
  • 4,540 POSTS
  • 2001 MAZDA 626
  • 104,000 MILES
The car idles rough when in gear, idle has been adjusted but still idles rough. I saw a previous answer about cleaning the throttle body and the IAC, would this cure the problem.
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Sunday, January 5th, 2020 AT 6:51 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Try cleaning it and see what happens and also go here: https://www.2carpros.com/articles/engine-misfires-or-runs-rough
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Sunday, January 5th, 2020 AT 6:51 PM (Merged)
Tiny
TOADSTER1971
  • MEMBER
  • 1 POST
  • 2002 MAZDA 626
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 136,000 MILES
My car idles rough normally and it takes off sluggish what are some tips that could be performed to fix this problem. The plugs and wires are only 6 months old.
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Sunday, January 5th, 2020 AT 6:51 PM (Merged)

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