At idle the engine is surging up and then back to idle

HOW HIGH IS TOO HIGH RPM? (NO TACHOMETER?)
NO ENGINE TOLD OF MANY SO ANSWERS NOW GO GENERIC.
ALL ENGINES CARS, FAST IDLE LOCK IS BASED ON ECT SENSOR COOLANT TEMPS, AND COLDER IT IS THE FASTER IT GOES.
EVEN BY EPA LAW USA.
cold is 1500 to 2500 most cars. pcm logic hard facts. and tables.
fluctuates is super bad and dangerous A.K.A SURGE.
there is no pat answer to this, so many engines.
but here are some.

• IAC BAD. (IDLEAIR CONTROLEER VALVE) THERMAL
• ISC STUCK (IDLE SPEED CONTORLLER ELECTRIC.
• THERMOSTAT BAD, OR MISSING.
• ECT READS WRONG/ Most times see 1 line up.
• VACUUM LEAKS FROM VAST THINGS. (YES VAST)
• PCM IN LIMPHOME MODE NOT SCANNED AND IDLE CONTROLS 100% OFF LINE. AND SURGING.
• GROSS MISFIRING.
• GROSS LEAN BURN AND SURGE.
• other sensors failing hard and you not scanning PCM miss the boat.

.

The power brake booster runs on engine vacuum to provide the power assist. Since it surges at idle and you mention that hard brake pedal application causes the surge as well, along with the knowledge that the engine was recently replaced, I'd be looking for a vacuum leak somewhere.

If you had the engine installed by a reputable shop, you should take it back to them assuming it started doing this just after the engine was installed. If they have a smoke machine for locating vacuum leaks, that should make the search even easier.

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)

Idle air control motor. Clean it or replace it.

Ecu temperature sensor. Or idle control valve. I would start with the sensor since is only a few dollars

WHAT COUNTY???????????????????????????????????
hot engine, cold or both?/// a key question
WHAT ENGINE: ( usa list , ONLY)
2.2 L F22B I4
2.2 L F22B6 I4
2.3 L F23A I4 2.3 L F23A7 I4
the fsm covers this. hose thing.... all hose can be followed,
unless its axle vents on 4wd cars. (they go no where, special)
is surging the only symtom.?
no engine stalls.?
lack of power in any gear or hills.
no misfiring?
no flooding.?
starts easy/?

ok what is surge, that it really is idle controls bad or out of comtrol
1: idle controls bad. (isc dead?)
2: or engine is running way too lean, and ISC cant keep a grip on idle.

easy huh?
and #3 misfiring so bad the idle controls fail.
are all fuses good?
is the CEL lamp glowing , then goes out running engine.
no , the scan it. or do the flash DTC codes trick.

a good tech ,first finds out if the engine holds closed loop status
hot. at idle, if not, he fixes that first.
as he/she knows that idle is the last thing to work right, in EFI.

one other way to get surge is air leaks (aka. vacuum leaks)
if the engine (unsaid option) is MAP based, it will race fast
with any vacuum leak, and it dont take much of a leak
to make the ISC idle controller go nuts.

so repost , hot/cold both questions
and actual RPM numbers. we need numbers
is the dash Tachometer missing. ????
and what engines in car.

all lean engines surge.
all ISC systems can surge if out of control , not all do this. just 1/2
if the ISC surges, it means its trying hard to gain back control.
this is called a software servo loop.
The ECU is programmed to hold rpm near say 800 RPM hot.
and is very digressive in that role.
I have a 91 car that will surge, with any vacuum leaks.
(by design) depends on which engine.

hot engine or cold, or both???????????????????
surging or hunting idle is complex but for sure the ECU has lost control of idle speed , there are 2 macro reasons, (vast causes)
1: ISC is stuck (in non SAE terms, the idle speed control valve ) is stuck.
2: engine is missing cause the ISC to GO NUTS trying to control
idle and can not.. the 50 reasons, , have list ask.
we do tests to find out if either or both the above are bad.
do you want tests, or guessing>???

(which engine. ?????)_
as always check all fuses. and scan the ECU for DTCs. first.
there is no need to fix OBD2 cars like 1980 cars (hard rows to hoe)
it can be 2 problems this......
lets fix the easy one first. idle. (and hope for moving issues to fix for free) it idle car is parked, so is 100% safe tests. (not moving)

so it surges at idle.
questions.?
1: hot or cold or both.?
2: low extreme RPM and high at idle.
400 to 800 RPM? surge.
1000 to 2000. what? numbers tell the tale.

first off, idle is the last thing to work right on any engine.

there are many ways to find what is wrong.
fix idle surge way.
i make sure that the engine holds closed LOOP status.
hot idle, if not , i fix that first.
keep in mind if not in closes loop and lean
all lean engine surge and guess what the ISC will add to the surge
because idle is way too slow, so the ECU open the ISC wider
and wider and its still too slow, so the ECU resets the ISC
this cause a worst hunt (surge)
the ECU adjust spark time and the ISC to get 800 rpm. (spec. look up yours)
so say , i see closed loop failed, OBD2 scan page 1
i look at the O2 sensor an its stuck lean.
if stuck lean and it really is (o2 bad?) then its lean and it will surge
.
examples.
1: i find 02 sensor bad, (only effects idle and light steady cruise)
2: i find fuel pressure at 15PSI , hopeless low .
3: i find all injectors clogged. and lean.
4: with MAF EFI, ( i find maf reading too low, (dirty) causing lean.
5: if MAP car, i find the MAP stuck hard high vacuum causing lean.
not knowing what engine you have or the idle hot surge range,
i can not go farther.

the driving bog tells me,
1: scan the ecu ,get DTCs; do that first.
2: clean MAF if car has one, ask how..
3: check fuel pressure.


my guess is engine is lean. and starving for fuel all the time.
cept WOT.

MAF cars starve for fuel if there are any vacuum leaks.
MAP cars do opposite, they race fast with 100% full power.
MAF is air density EFI
MAP is speed density EFI.
apples and oranges diffr. so what engine.

Surging idle is usually caused by vacuum leaks. Ford 5.4L engines are notorious for the PCV hose elbow leaking back behind the throttle body.

The PCV tube goes from the right (passenger) side valve cover and loops around behind the valve body. Inspect this hose on BOTH ends.

Sounds like a possible vaccume hose leak?