1997 Protege LX idle speed hunting

[Baralheia]

Hey, I'm new here. I drive and work on my Dad's 1997 Mazda Protege LX with the 1.5L engine and 5-speed manual transmission, and it's got 104,000 miles on it. I've noticed that upon starting the car, or while driving, when letting out on the clutch to begin braking, the engine will stumble, and the tachometer needle falls to just above zero. Most of the time it'll recover in a couple of seconds, but sometimes I have to give it a little gas to bump the idle speed back up to where it normally is (around 450rpm or so). It has also started to do this when driving with the A/C on... When the compressor clutch disengages, if the engine is at idle (car coasting or stopped), the engine RPMs will fall as described above. It's bad enough that when the A/C compressor kicks back on, it can stall the engine!

My Dad has not taken the best care of this car, except for regular 5000 mile oil changes, and periodic air filter changes. I know it needs a tune-up, and a new fuel filter (both the ignition system and the fuel filter are original to the car ), but until just yesterday, the engine didn't idle funky like this. Anyone have any tips for how to diagnose and repair this problem?

 

[BlueMonsta]

under ur hood... it should state the regular idle engine RPM...
after that u can get it tuned so that it does IDLE at that rpm.... i had to move mine from the regular 750 to 800 cause the steering wheel was shaking.

ur abnormal engine idle speed causes could include

engine has water (if u went to a car wash or something or just drove over a big puddle) <--- highly unlikely though

ur intake/exhaust values are clogged with carbon disposits <--- I think this could be a cause some tell me if im wrong

ur timing belt may need to be replaced, since u mention it happens when the A/C is turned on <-- again i could be wrong...

 

[Salmon_Rob]

also your intake piping might be dryrotted or cracked

 

[Baralheia]

Hmm... Makes sense. I do know that the car has occasionally been treated with Berryman's B12 Chemtool for fuel system cleaning, but after 100k, I wouldn't be suprised if the engine had some carbon deposits... As for the timing belt, how long do those generally last, and are they difficult to get to? And on the intake hose... Yeah, mine is cracked at the part that was supposed to flex... I was checking the air filter at the last oil change, and as I started to pull up on the air cleaner housing, there was suddenly very little resistance, and when I looked at the hose, it had nearly ripped in half. Right now I have it taped up pretty well with some of that high-temp duct tape that is sold for minor radiator hose repairs. I can't believe how expensive that sucker is new from the dealer... P/N B5C9-13 - "Hose, Engine Fresh Air Intake" - is $61.55 plus tax!

Also, to clarify a bit, the low idle speed condition will happen when the clutch is released, allowing the engine RPMs to drop from cruise speed down to idle. This will only happen about half of the time, and when it does occur, will happen regardless of whether or not the A/C compressor clutch is engaged. When the A/C compressor clutch is engaged, it is obviously harder for the ECM to adjust the engine idle speed back up to a smooth idle speed, so the engine idles poorly for longer than it does otherwise.

 

[Salmon_Rob]

yeah, there is probably a vac leak caused by the ripped hose, and vac escaping through leaks in the tape. id suggest either getting the part from the dealer or an aftermarket

 

[Baralheia]

yeah, there is probably a vac leak caused by the ripped hose, and vac escaping through leaks in the tape. id suggest either getting the part from the dealer or an aftermarket

I don't quite understand why that would cause the idle speed to fall like it does, though, since the hose in question is before the throttle body. If the hose was between the throttle body and the engine, then it'd make sense... but the 1.5L Z5-DE engine uses a metal plenum in that space.

Besides, it exhibited similar behavior before the hose ripped... the deal with the engine dying with the AC turned on, though, I found out was due to an overfull A/C system. Correcting that problem, however, still did not fix the idle speed issue.

I noticed there was a TSB released on the topic, but my Protege doesn't fit within the VIN breakpoints, so I don't know if it applies...

 

[Salmon_Rob]

its a question of vacuum in your intake. without it, the car doesnt get the proper air it needs, and so hunts for more air, causing your idle to fluctuate.

 

[txmzdspd2gen]

HEY..check you a/c compressor..thats exactly what happen to my car..mines an automatic but did the same thing...thought it was nothing till the smoke starts coming out...burned my a/c clutch and compressor..cos a lot to get it fix...also wanna check your coolant temp sensor...

 

[03.5MSP]

Mine did the same, I noticed that I was stalling out,(and the engine struggled with the ac on) so I went to the dealer and got them to adjust the idle speed.

 

[95prot]

Idle speed fix

It sounds just like a problem I had with my 95 Protege. My air intake hose was half cracked at the flexible part of the hose - and as a result the 1.5L would not idle at <1500 rpm or so.

It seems to me that the hole in the intake causes additional air that is not measured by the mass airflow sensor (which is upstream of the torn hose) to enter the intake, throwing off the air/fuel ratio. Your engine will probably sound like it's misfiring at idle rpm - mine did, when it didn't stall outright.

I solved the problem by buying an intact intake hose from a junkyard - $5. It idles nicely down at about 800rpm now.