Idle Surge Frustrations

[TomPrestin]

1996 X90, 16v G16b.  Stock engine, full rebuild. 2 years ago. Thorley Exhaust Manifold.  

Here’s the rundown as it stands:
For the first 6 months after rebuild engine ran well, no obvious complications, summer in midwest so average temperatures.  P0400 showing, been hunting for the cause with no luck since beginning.  Winter cold sets in and I begin to experience an idle surge.  Random, occasional progressively got worse until flatlining as constant with the following conditions.

At this point I’m running my OBD2 scanner pretty much at all times in hopes that it will give me an alert upon some circumstance which will point to the issue, but so far no luck.  Occasionally I’ll get an “Idle Air Control System” code.  Very rare, this has been happening for almost a year now and I’ve only gotten that code twice.  Still consistently getting the P0400 EGR code, no luck deciphering its root.

cold start idle holds at anywhere between 2500-3000 for 30 seconds to a minute, then drops and begins to surge
1800-2300rpm.  Once fully warm the surges increase in sequence slightly, and drop slightly 1500-2100rpm.  
Replaced the ISC; No change
Replaced the entire throttle body out of suspicion that the thermal IAC was defective; No change.  
Disconnect ISC; no change, 
Disconnect (electrically) any (and even all at same time) piece of the EGR system; No change.
Pinch test every possible line in and out of vacuum/EGR system; No change
Physically open EGR Valve; slight decrease in top and bottom of RPM surge.


Any thoughts would be REALLY appreciated.  Its excessively annoying as much as it is concerning, and has an unsafe effect on driving when the throttle surges and pulls even in gear if not on the gas pedal.


Thank you!!!!

[fixkick]

p0400 means the  EGR is bad, they fail 2 ways, one causes only silly error (stuck closed) failure mode  #2 stuck open and engine will not idle and stalls )
only a new one cures sticking, failure mode #2 EGR, is at random or every stop  light (sign) the engine stalls for #2 cause.  as you stop the car the ECU commads the EGR closed.

and fails.
OBD2 car USA>
know that the ECU only can tell you  if  sensors are dead, and not weak, with some exceptions, like Idle Air Control System” code. and P-042x errors CAT weak.

Unsafe at any speed,  (RALPH NADER 1965 do not look here) my joke this morning,
he'd have  cow on this car,  Toyota paid out billions in fines for this,  not suzuki yet and it is guilty.

the ECU on most these cars 80/95 love to surge, but not 96+ that Ive seen. but yes, sure can if some things happen.  (not ECU fault (LIKE VACUUM LEAKS DO)
to fix any car like this , this is what I do

1. clean the EGR or replace it, make sure it is 100% closed and not leaking air past the valve inside, then defeat it until car runs right again, !!..
   
2. to defeat it , pull the hose to it  and use a GOLF tee , in the hose so the engine can not suck ILLEGAL unmetered air.  so easy to do this.  until it runs right again !
   
3. EGR is now 100% defeated so we can focus on all other things,
   
4. also rule #1 , is OBD2 may take 2 or more driving cycles to show DTC errors so always use the PENDING button on the scan tool, code readers cheap are junk here.
   
5. The first thing to do is,  test the MAF< output at keyon not started, and at idle and then gun the throttle , 3 tests seen here. ( I clean it first with MAF cleaner or carb cleaner)
   
6. make sure there is no damage at all to the main huge induction pipe from TB to MAF,  mine had bad boot gaskets and the aluminum pipe there had broken mount and was sucking air via this huge, 1/2" square cracked off mount. (day1, found 1996 tracker) 
   
7. next is fuel pressure, the FPR loves to fail this old, and fuel pressure at idle and key on only must not be out of spec, and the 6PSI   pressure drop keyon only to idle.
   
8. ect tests good, hot 300 ohms hot. hot engine key off, unplug ECT connector , using unlock tab, reads 300ohms for 180f,  must or near.
   
9. all 3 throttle cables must have 1/2" slack (throttle, A/T kickdown , and cruise if opted)
   
10. foot off throttle the TV valve is closed 99.9%, or at .0005" gap, if not see #9 or find  some bad guy secrewed in the never touch stop screw, at TV base gap.
    
11. This is just a starting set of checks , lots more can be done. but first get the EGR defeated and the MAF working first.
    
12. Do not ever use EBAY or amazon $15 MAFs ever. they are all Crap.  CLEAN YOURS FIRST THEN TEST IT. NEXT
    
13. WATCH OUT FOR SHOT GUNNING PARTS,  as this can add one more bad part to the mix and super hard to diagnose that. (many parts sold now are crap  from china)
    

[fixkick]

the surge means AFR is way way off
AIR fuel ratio.
that ISC unplugged would stall any good engine hot.
when hot the this engines, has no air supply at all with ISC unplugged, that is a fact, (ISC closes at 150F) if thermostat is bad, well that be first, things first.
hot engine air, all
1: TV (zero air here) if cables slack there must never be air flow here, TV = the brass throttle valve in the TB.
2: IAC closes at 150F, but hot engine is 180f or more, your scan tool tells you ECT reading 180f is normal not 140f ever hot. easy no, just glance at OBD2 ECT readings LIVE.
3: ISC, is electronic idle control and is 800RPM hot, unplugged it closes 100% by its own springs inside, and all air ENDS NOW and the engine must stall.!!!!

if not we then find out why.
surely you checked ECT first for 180F hot. if fails the 7 year old thermostat is bad, as all are that old so says president of STANT in his youtube tour.
there are only 3 sources of fresh air,
all closed, there is no AIR , and engine must stall.

other oddities, ?
the EGR gasket is metal, if missing sucks huge fresh air
as does all 4 injector base cushions all cracked and old.
im sure your 96 has the EGR monoport TB, right> ? 95 is quad port , and 96 introduced the new TB with top center EGR mono port EGR injector .

you did not state how car drives, besides, idle creap, how is engine power in all modes of driving, slow /fast, hills, accel? and that> power wise engine power ?

[fixkick]

the EGR has 2x , 6mm bolts holding it on and way way hard to reach back of intake man.
best is not to force them
use penetrating oil there and best is spray there hot and let engine cool and this fluid sucks in hot to cold
and then jog them, CCW,CW ,ccw, jogged, to get them lose gently with about 5 ft./lbs torque, 100inch.lbs is spec, so go easy not stamp these screws.

let me know how you do
and do ask questions,
the tests are mostly simple, using normal car tools.