Flat spot when accelerating or climbing

[NopeNope]

Hey Y'All,

 

My '91 Subaru Legacy seems to have a flat spot (I believe that's the correct term, but I'm still not fully versed on some of this stuff). Generally occurs when accelerating (usually somewhere between 2k and 3k RPMS) or climbing a hill: I'll depress the accelerator but nothing happens (RPMS don't climb, no speed increase). If I keep giving it gas it'll eventually surge and begin accelerating relatively rapidly (except when climbing hills, usually I just have to limp to the top); sometimes it'll surge then return to lagging, then surge again. Doesn't seem to demonstrate this problem above 3k RPMS, though every once in a while it does below 2k.

 

What I've done since the problem appeared. I've changed or had changed: spark plugs and wires, intake manifold gasket, O2 Sensor, & Throttle Position Sensor.

 

Originally it had the flat spot intermittently and would occasionally lurch or misfire on the freeway (and I'd get an occasional ECU Code, though at the time I hadn't pulled the codes), so I took it to a mechanic for diagnosis (this is about a month ago at this point). They told me the sparks and wires needed replacing and I had them replace them. The problem persisted, so I took it back to the mechanic; they diagnosed a bad O2 sensor, bad intake manifold gasket, and bad fuel injector seals. I replaced the O2 sensor and intake manifold gasket. Car was 10x worse (had no power at all, couldn't maintain speeds above 30 or 40, etc.), so I took it to another mechanic who pulled codes for a bad O2 sensor and replaced that (the O2 sensor I'd used was cheap and the mechanic said it likely was poor quality). Now it's back to the condition I originally described (flat spot under load). After doing some research, I replaced the TPS (twice, using TPS's pulled from junk yard subarus), and it's appeared to fix the problem very briefly, but within a few hours the problems return. It has been giving occasional ECU Codes, specifically for the O2 sensor (which I have trouble believing is bad since it's been replaced twice), and just yesterday it threw codes for the TPS and Idle Switch.

 

I'm at a loss. Still need to replace the Fuel Injector Seals, but haven't been able to get them out and scared of breaking them. Wondering about the possibility of a wiring issue since the O2 Sensor and TPS have both been replaced multiple times and continue to throw codes. Saw one post suggesting a knock sensor might cause such a problem, but not stoked to pull out the intake manifold again unless there's more indication that is a problem. Anyone have any thoughts?

 

Thanks.

[NopeNope]

also, forgot to mention: the green power indicator light frequently flashes (16 times?) when I start the car.

[lstevens76]

Ok, why are you going to pull the manifold to pull the knock sensor?  12mm socket + extension it should come right out and put the new one in.  It might take a little finagling to get it in, but a lot faster and easier than pulling the intake.

[kuw4subie]

I'm contending with the same symptoms of the flat spot (recent) and 16 flashes (for a few months now) on the same EJ22 engine (94). Are you having any other symptoms beyond these? I've also dealt with worsening gas consumption, smell of unburned fuel when accelerating under load (slowly as to not hit the "flat spot"), fuel smell in changed oil and on all 4 spark plugs. Resuming efforts on replacing Fuel Pressure Regulator momentarily (little phillips screw are a huge pain!). Fingers crossed here & wishing you luck too.

Edited September 19, 2015 by kuw4subie

[subakev]

Have you checked your egr valve

[kuw4subie]

There's no EGR valve in the earlier(90-94) 2.2L Legacys.

[subakev]

Well that show ya what I know

[kuw4subie]

Good thought though. I only know this because of going down the same road with researching these symptoms and having Subaru parts dept confirm the lack thereof.

[Bushwick]

How many miles are on it? Really high mileage engines can have excessive wear on the cam lobes, which, as you guessed it will alter the powerband creating a dead spot in the power.

 

 

Given you are having electrical issues though, I suspect there's an issue with a wiring harness, plug connector, corrosion in a connector, etc. Wouldn't hurt to inspect and better yet just replace ALL vacuum lines. This means ALL the lines going to the assorted sensors under the intake runners AND feeding the throttle body. Do ONE hose at a time to avoid reattaching incorrectly. Old hoses can crack, split, soften, become loose on their respective nipples, etc. and while "appearing" OK to the quick eye, can actually be bad. 

 

It costs $10-15 to replace all of them. If they used jump sizes i.e. a 1/4" T's off into an 1/8 " and 1/4", you have to get creative and make something work or try and find OEM if they still make it.

 

While replacing the vacuum lines, keep an eye out for odd looking wiring. At the very least, you spent $15 to get the engine in better shape. Most people do tune ups and completely neglect the hoses which can cause lean conditions, rough idle, issues with sensors that rely on them, etc. Reset the battery after replacing them. Hopefully it helps. Keep an eye out for chewed wiring, and pay close attention to the wiring feeding the sensors you've had codes for.

Edited September 20, 2015 by Bushwick

[kuw4subie]

Hey NopeNope.. Thanks for the msg'd inquiry. To answer your question, my symptoms were due to a couple of issues. My timing belt being not properly installed was the big thing. I have the non-interference EJ22, thankfully, and while I did an intial count of the teeth for each zone, the belt must've crept just enough to throw the timing way off. I re-did the install with mini c-clamps to hold the cam pulleys in place, the 2nd time around, and meticulously counted teeth as needed. Also replaced the hydraulic tensioner adjuster, as I had milder yet similar symptoms before I even replaced the timing belt after a water pump failure. If you have a high mileage subaru, I'd take a close look at your adjuster as well. The beginnings of a hydraulic fluid leak are barely discernable, as I had just a bit of sheen on the gasket seal that surrounds the piston for such- but not like an obvious wet leak. But you gotta figure that any leakage can compromise the strength of the piston's pushing power. When I took the old one off and it wasn't under load, the piston came out a bit further. So I got it all back together and it ran smoother than it had in over 6 months! Then I was able to figure out I still had leaky head gaskets from the resulting over-heating from the water pump failure (on the highway). So I'll be putting it all back together again this weekend and trust I have the timing belt install process down-pat now! And also shiney resurfaced heads too... But that adventure is noted in another post. I love this forum. Y'all rock!!!

Edited December 11, 2015 by kuw4subie

[Fairtax4me]

and just yesterday it threw codes for the TPS and Idle Switch.

 

TPS needs to be adjusted.

 

Other symptoms match up with a bad knock sensor. Easy to replace. Usually under $20 on eBay. Last ones I got were $8 each.

[crazyhorse001]

If you have a flashing power light you have trans issues. You need to read the trans codes too. Do a search for that.

 

The key is the lurch, its not downshifting properly. A tps not set right will do this. Next time it bogs try this. Let up, then "punch" the gas. If the power light comes on, and the revs jump up, you can bet the tps has the "dead spot". They get one around 45-60 mph, as thats where the pedal rides at cruising speeds.

 

Dont overthink it. If yoh have a cel, for the tps, and a flashing power light, that comes back to the same thing, you have either a bad tps, or bad wiring to it.

[7sisters]

I've experienced a similar situation with model and year after driving it for 100K and 10yrs.  Recent, fuel filter, tune up, good things, although still symptomatic.  The experience could be the catalysts which I've researched to either over heat/quick temperature change, are damaged, or O2 sensor replaced/MAF cleaned  (then throtle and each connection, make worse?)  The check engine light is out thus no codes until flashes seen, (never seen any of that in driving) test similar to emissions, or replace them with used that supposedly can be checked as good and look inside current catalytic converters after change to see if MAF/O2 cause.  May not practice what I believe although I do care about the earths atmosphere.  Simply like to see if back pressure is created and keep traffic happy although santas elves are busy/shops owners need to put priority to those with more money.  Thank you all for any feedback...