1998 Subaru Legacy Outback, Hesitation off idle

Hey guys,
Hope you're all well. I have a 1998 Subaru Legacy Outback. When the car is fully warm, I'm getting a hesitation when taking off from a stop. Momentary drop in RPM's, no power, and then it goes. It's never been so bad the engine has stalled. Only occurs when warm. Additionally, I can feel a slightly uneven power output (unsteady RPM's) at very low speed after one of these hesitation events...like when I'm creeping along in traffic at 10 MPH.

No codes in memory. Doesn't lack power at higher RPM's under load, which leads me away from a fuel delivery issue, at least initially. Total fuel trims are running at around -10% or so at idle, and positive 10% or so at highway cruising. Fuel trim numbers seem to mimic a dirty MAF, but the overall trim numbers don't seem too crazy.

What else should I be looking at with a symptom like this? Looking at the front O2 during the event reveals that the engine is going lean during these drops...but I'm figuring at this point this is a symptom, and not the problem (like it would be if this were a fuel delivery issue...but again...only running slightly lean at cruising speed, and rich at idle, with no real noticeable loss of power under load).

Any ideas appreciated! I'll get some time this weekend to dive into diagnostics, so looking for some ideas before I dig in.
Thanks,
Rob

What's the mileage on the engine?

I actually ran into a nearly identical Legacy a couple years ago with the same problem, never fixed it So, I'd love to get to the bottom of this one!

One thing I thought of later (and wish I'd have tried) was removing the knock sensor from it's mount, wrapping it in a rag, and rechecking for the hesitation. The idea would have been that the knock sensor (or an actual knock) was causing the PCM to falsely retard ignition timing.

You could also try scoping the MAF during a snap throttle to see if you get that characteristic 'spike' on initial throttle opening. Though, I agree with you, this doesn't necessarily match a MAF problem

Caritech, it has 176,000 miles.

Tyler, thanks for the input. Is there a section in SD's book that talks about the characteristic MAF spike you mention?
Regarding the knock sensor, I'm not hip to "known" good operation regarding timing advance/retard on this system. I hooked up my scan tool and very briefly watched the timing advance PID while driving. Taking off from a stop, timing retarded, very momentarily, to a reported -2.2 degrees. There was a very minor hesitation there. Wasn't able to recreate the symptom at it's "finest"...but I'll keep trying. Does that -2.2 degrees supply some evidence for your knock sensor theory? If so, I'll proceed with removing and towel-wrapping it (I assume you mean by this to have it plugged in but un-installed and towel wrapped to deaden its sensitivity?).

Rob

RobBrown wrote: Caritech, it has 176,000 miles.

Tyler, thanks for the input. Is there a section in SD's book that talks about the characteristic MAF spike you mention?
Regarding the knock sensor, I'm not hip to "known" good operation regarding timing advance/retard on this system. I hooked up my scan tool and very briefly watched the timing advance PID while driving. Taking off from a stop, timing retarded, very momentarily, to a reported -2.2 degrees. There was a very minor hesitation there. Wasn't able to recreate the symptom at it's "finest"...but I'll keep trying. Does that -2.2 degrees supply some evidence for your knock sensor theory? If so, I'll proceed with removing and towel-wrapping it (I assume you mean by this to have it plugged in but un-installed and towel wrapped to deaden its sensitivity?).

Rob

I thought there was an example in the Airflow Sensor section? I'll double check when I get home. 'Till then, this video has a pretty good example of the throttle opening spike:



The timing retarding that much... Might be onto something? And yeah, I meant to say keep the sensor connected, my mistake! I suppose you could try unplugging it, too, but I'm not sure that the PCM would advance the timing with a circuit code set.

What's the most a knock sensor input could retard timing?

Or perhaps a better question would be, what is a typical range that a knock sensor input will retard timing?

Have you cleaned your throttle body/plate?

Noah wrote: What's the most a knock sensor input could retard timing?

Or perhaps a better question would be, what is a typical range that a knock sensor input will retard timing?

Good question I have seen other makes retard timing quite at bit when they detect excessive knock, but I figure the exact range will depend on the application. I'll see if Mitchell will tell us on Monday.

Have you cleaned your throttle body/plate?

Oooooh yeah, nice thinking. Keeping it simple

Oh yeah, Section 12 Page 11 has a good example of a MAF spike on throttle opening. It's on a digital MAF, and the Suby uses analog, but the principle is the same.

Hey Rob!
When was the last time your timing belt was replaced? May be worth a quick check! First . I had a similar instance with a rover with the K series engine once! And turned out to be out one tooth on the intake cam!

Any scope captures of what's going on, in ignition, at the time of hesitation and/or general operation?

mal16pindlc wrote: Hey Rob!
When was the last time your timing belt was replaced? May be worth a quick check! First . I had a similar instance with a rover with the K series engine once! And turned out to be out one tooth on the intake cam!

Easily done on the Rover K16 especially if the Laser Timing Lock Tool is used things can go wrong as the tool lines everything up 1/2 a tooth out, that the crank pulley mark is impossible to see dosen't help. There is a trick for setting the K16 up, as the engine is timing is set at half stroke all the piston should be at equal height in the bore. I have four wooden rods (actually wooden knitting needles) that I put down the plug holes. As the rods are all identical length if all 4 are sitting at equal height the engine is at mid-stroke.

Andy.MacFadyen wrote: Easily done on the Rover K16 especially if the Laser Timing Lock Tool is used things can go wrong as the tool lines everything up 1/2 a tooth out, that the crank pulley mark is impossible to see dosen't help. There is a trick for setting the K16 up, as the engine is timing is set at half stroke all the piston should be at equal height in the bore. I have four wooden rods (actually wooden knitting needles) that I put down the plug holes. As the rods are all identical length if all 4 are sitting at equal height the engine is at mid-stroke.

Dude these Rover cars sound like total crap! :lol:

Dude these Rover cars sound like total crap! :lol:

They really just had a problem with change to the casting method of the 4 cylinder engine block they made 3 years into the production of the engine that introduced a cylinder head gasket issue. The change was made just before BMW bought the company from BAe by the time the issue came to notice as a serious matter BMW was having a major boardroom battle over the Rover purchase and was disinclined to invest in development to fix the problem instead poured money into developing what became the BMW Mini which Rover were developing as a Rover model.
Skip a few chapeters and eventually Rover ended up in Chinese ownership and a very simple change in the head gasket design and material solved the head gasket issue completely, and engine and car is still in production today.
The engine itself was pretty sweet introduced in 1988 it weighs next to nothing and the 1.4 litre produced 104 bhp Din and the 1.8 vvc version 158 bhp Din.

Before BMW bought Rover Honda had a very close cooperation with Rover in fact Rover built Hondas for the EU market and for some non-EU markets the certain Rover models were built in Japan by Honda. When BMW bought Rover without their consent Honda were extemely upset and with drew all cooperation. The irony is if Honda's advice had been available for the head gasket issue the problem would probably never have occured.