Unexplained intermittant hesitation 99 OB

The car: 99 OB wagon, 5 speed MT, 209k miles. Both O2 and knock sensors replaced around 170k miles due to failures that threw codes. Plugs and wires also replaced at around 170k miles. MAF replaced with good used one around 190k miles. No recent check engine light or codes.

 

Twice in the last few weeks the car hesitated, by which I mean that the car had barely enough power to keep moving. In both cases the engine had only run for 1-2 minutes after a cold start on 85 deg F days. The first time I was going 30ish mph when it occurred, then the car slowed to 20-25 mph and could barely keep going for about a minute, then the performance recovered. The second time I was leaving a parking lot with several stop signs, and the car could barely get out of it's own way. This incident lasted 1-2 minutes.

 

My first thought was the fuel pump which I believe to be original, but after some searching I've seen references to engine temperature and a few other sensors.

 

So, I don't want to start throwing money in the form of parts hoping the problem doesn't come back, but I also prefer to not get stranded. Being rescued can be expensive too.

 

Thoughts?

Pay close attention to your gauges during these hesitation events because dimming of the lights & dropping needles indicates an alternator problem.

No dimming of the lights. Voltages are good.

 

The alternator was replaced less than a year ago after a Subaru dealer supplied remanufactured alternator failed in less than 30k miles.

 

Battery, which also tests good based on the "cranking the engine without any problem" test, is less than a year old. It was replaced at the same time the alternator was replaced, likely done in by the alternator only charging it intermittently.

 

Battery and alternator cables are also good. I will probably check all grounds for corrosion and bolt tightness, although I don't believe the problem lies there.

Sounds like a faulty MAF sensor since it's a load variant problem. That and you bought a used one. Maybe try cleaning it and also remove your IAC and clean it as well.

In 15+ yrs of automotive repair I've never replaced a coolant temp sensor on a Subaru. They don't fail after or at all.

Sounds like a MAF problem. I have had the same issue. I would check your fuel pressure after you clean the MAF.

http://legacygt.com/forums/showthread.php/strange-hesitation-202183p2.html

 

I have a similar bout with hesitations on my 99 wagon... Maybe something will help you in that thread.

 

Mine is down to a tolerable level now but still does it.

I just remembered something that may be important... Or it may just be coincidence... Both times the car hesitated and lost power it had been raining hard. But the problem disappeared despite the rain.

 

So today I cleaned the MAF and replaced the fuel and air filters, which were both replaced at 185k miles, or 24k miles ago. PCV valve and spark plugs were also replaced 24k miles ago. Fuel filter did not appear to be dirty or contain any water based on the clean gas that came out.

 

While I was cleaning the MAF I also cleaned the throttle plate, which was really not all that dirty. (I did not clean the IAC because I had no gasket, but symptoms make me think it's not the IAC. Power loss was when engine was no where near idling condition.)

 

Due to the problem's possible association with rain I took apart and cleaned all ground wire terminations in the engine compartment, including the two hard to reach ground connections between the engine cam box and the frame rails.

 

So we'll drive it a while and see if the problem comes back. I'm still suspicious of the fuel pump.

Met someone today that suggested I check out the catalytic converters. If they're coming apart from age and vibration they can temporarily block the exhaust. I would think that you would have to shut off the engine to eliminate the exhaust gasses that hold the converter chunks in their blockage position, and my problem went away when driving with the engine (barely) running. Still, something to check out.

Mine WAS throwing cat codes but hasn't done it for months. Code P4020.

 

Beyond dropped valve guides that's the last thing for me to check...

http://www.thesubaruforum.com/showthread.php?978-Legacy-station-Brighton-99-delays-(hesitation)-on-accelaration

Ouch nothing like ECM replacement... This is the precise problem I have....

With the clean MAF and new air filter the car does run better.

 

There was a hard rain this morning, but no repeat of the loss of power.

Weeks later......

 

I haven't been driving the car much because my daughter's Saturn had Passlock issues (Curse you GM engineers!) and I "had" to leave the OB for her as a back up. She finally mentioned a loss of power when first driving the car.

 

Now the car suffers from a 15-30 second episode of power loss almost every time the car is driven. And when the episode is over it's not repeated until the next time the car is driven.

 

I did finally test the fuel pressure. Pressure is a solid 38 psi with the pressure regulator vacuum hose disconnected. With the pressure regulator vacuum hose connected fuel pressure tracks the manifold pressure, with a pressure of 25-28 psi idling. Pressure takes minutes to bleed down.

 

I may extend the hose to the fuel pressure gauge so that I can tape the gauge to the driver's side outside mirror and observe pressure during one of these loss of power episodes.

 

So... While I plan to test further the fuel pressure, I no longer strongly believe that it's a weak or dying fuel pump.

 

Sometime I wish the car would just throw a code so I could more easily diagnose this problem. It doesn't help that, until September, I can only drive or check out things on the car except for weekends.

 

Thoughts?

Get a new MAF and a Walbro!