I have a 1997 subaru outback that surges like a misfiring plug while driving 55 to 65mph and as the surge happens the tack increases 100 to 200 rpm as if the transmission were slipping for just an instant and re engaging or possibly between gears or possiblybetween 4th and lock up converter? possibly a lock up tourqe converter problem if these cars are so equipped? No warning lights are present and no codes in the computer

scan it yet, a FULL OBD2 scan? why not?if not.
now is the time...


the 1.8L J18 engine, same as the older Sidekick sports used.
same engine , know it I do.
its the precursor to the J20, (j18 is under-stroked, j20 with many of the same parts. inside)
the back fire means explosion sound to rear end of tail pipe?
yes, that is rich raw fuel in the muffler. plus Air.
if there are exhaust cracks and then engine goes rich this can happen( a double failure)
if how ever BACK means, back to the Air cleaner, then that be different reasons, (timing)


engine now starts.?>
engine runs.? has full power, car drives ok.?
JUST?
SURGE,. you never said.

does engine get hot? 180F or more and holds (y/n)
ever get a full tuneup this decade,
most dont so i tell you, bad spark causes, misfire and raw fuel is wasted to the exhaust and sure can go boom.

surging usually means the ISC can not control idle
so it hunts, (a suzuki software engine on all 90s engines)
PCM errors, inside it. (dangerous too)
a modern car, if idle goes too high the ecu commands the ISC closed
and holds it there solid, (safe way , no hunting allowed now)]
so if this car surges, it's , lean engine.

some think flooding is surging.
and this is usually below 800 rpm,
almost stalls, then corrects, seems like a surge.

next time post rpm range of surge.?????

do not drive a surging engine, do not be like Toyota drivers. (over 800 rpm hot)

surge from 400 -800 rpm is class 1 surge.
surge from 800 to say 2000 RPM is class2, and very dangerous to some folks, (key off, get car fixed, tow it)

The ECU controls idle speeds. (hard coded servo controls)

Is the car equipped with the Automatic Transmission?
If so, read on.
Otherwise skip to the end and answer some questions.

POSSIBLE SCENARIO:
I have observed a condition where my car surges slightly when the torque-converter clutch (TCC) cycles between lock and unlock when driving on an uphill grade.
First some basics and history that will explain why the TCC is used.

Engine, Torque Converter, TCC, and Transmission relationship--
The TCC allows for a solid connection between the engine and transmission which allows the input to the transmission to rotate at the same speed as the engine.
Without a TCC, there is slippage between the engine and automatic transmission. The slippage is greatest at low engine RPM. That is what allows the engine to run with the automatic transmission in gear, like when you first shift into gear or stop at a stop sign. When the throttle pedal is depressed, the engine RPM begins to increase and the torque converter begins to slip less and less the more the engine RPM increases. The car moves. But even at cruising speeds the torque converter slips slightly. Engine RPM is greater than transmission input RPM, which is realized as slight decrease in fuel efficiency.
When acceleration is complete and a constant speed is being maintained, the engine power output is reduced to the point where the TCC can engage and eliminate any slippage between the engine and transmission. If the car has a tachometer the engagement of the TCC can be verified when a slight reduction in engine RPM observed without a corresponding change in vehicle speed.
One method used to test the operation of the TCC is as follows:
Find a flat section of road where it is safe to perform the test.
Reach a steady speed and keep the gas pedal depressed with one foot. While observing the tachometer (or listening for an increase in engine RPM), with the other foot depress the brake pedal enough to activate the break light switch but not enough to engage the brakes. When the brake light switch activates, the TCC receives a signal to disengage. With the gas pedal being held steady, release the brake pedal and the engine RPM should decrease when the TCC engages.
Old cars with Automatic Transmissions did not use a TCC. I believe the TCC was put in use in an attempt to increase fuel economy.

MY EXPERIENCE WITH SIMILAR SYMPTOMS
The condition that causes that issue on my car is this:
- A slight uphill grade increases the load on the engine.
The car tends to gradually slow and it is necessary to depress the gas pedal to maintain speed.
- Depressing the throttle pedal (manually, or automatically with cruise control engaged) signals the torque converter clutch to unlock when the load increases slightly. (A more drastic load increase would signal the Transmission to downshift to a lower gear.) The corresponding increase in engine RPM and output is enough to compensate for the reduction in speed. When the vehicle speed, engine RPM, and throttle position stabilize to the point that the TCC will engage and the engine RPM will reduce in correspondence with TCC engagement. Now, if the road conditions have not changed, power output is not enough to maintain vehicle speed. With the increased load caused by full engagement between engine and transmission, and the cycle (surging) repeats itself until the road conditions change.

Does that help?
If not:

QUESTIONS
Please define the symptoms.
What are the road conditions when the surge occurs? (A slight uphill grade?)
What is the frequency of the surge?
Does the engine power output have a noticeable surge?
Is there a speed change related to the surge?
Does the tachometer move up and down with little or no change in vehicle speed?
Are all instrument indication in the normal range?
What else has changed?

Good luck!

That's some mileage you've got on the car. It has obviously been well maintained?

Car engines do surge, Deb. Next time you stand next to a bus or truck just listen to the sound of the engine .. an occasional surge and change in the engine note.

It doesn't sound as if you have anything to worry about

possible computer problem

check the spark plug or injector or coil

how about the sparkplugs and the wires, you may need to tune it up, sounds like it misfires under a load, i would start with a tuneup and go from there. the other thing that come to mind could be a bad coil pack, or low fuel presusre,if you have the computer scanned and pin point the cylinder or cylinders misfiring you could try switching coil packs or plug wires to see if the misfire move and if it does you would know if its a wire coil or plug, but if its been a while sence you had a tune up i would start with that.

It may be one of two things. A intermittent problem with the idle air control sensor or the throttle position sensor. Have the cars computer scanned at a local auto parts store to see if there is a pending code that has yet to turn on the check engine light.

I would do that plus test the plugs, wires and coil pack.

replace your sparkplugs, replace your leads to the sparkplugs, get a mechanic to tune your car.

no the surge that high is not normal a normal cold start should go to around 1500 then settle slowly to 750 or so but this all depends on engine temp and ambient air temperature