[Diy_efi] Dead cold rough idle

Fri Feb 17 18:59:19 UTC 2006

Hunting at idle might also be caused by wrong control parameters for the 
  idle speed stabilization or too early ignition (then the engine 
response to a change of the idle air valve will not match the control 
parameters and the system will oscillate).

Idling problems at a cold engine can usually be avoided by a good intake 
air preheating and, of course, heating of the intake manifold. Some 
european TBI engines (for example, the 1,6 and 1,8 l engines of VW 
Golf/Rabbit from about 1988 to 1997, with Bosch Mono-jetronic or 
Mono-motronic TBI) have an electric PTC heater of about 300 W directly 
below the TBI unit. This heater is controlled either by a thermostat or 
by the ECU, so that it is switched on from a cold start and until the 
coolant temperature has reached about 50-70 ?C (120-160 ?F).

Best regards

Torbj?rn Forsman

Lee M. Lemoine wrote:
> Usually when the engines hunting like that your at the edge of a fuel 
> map... meaning that either your 500 rpm table is set too rich, so the 
> extra fuel makes the engine rev a slight bit... but the 1000 rpm table 
> may be too lean, so it falls back again...  (rinse, repeat, etc) and 
> viola -- a rough idle.   The other things that I've seen related to this 
> would be the air intake temp compensation (too much / too little) or 
> even coolant temp compensation, although you say it seems to do that 
> even after warmup.
> 
> Most EFI systems use a 'cold start enrich' map, which lasts the first 30 
> seconds or so, and fades out gradually.  What this means on a cold 
> engine is an increased amount of fuel -- causing it to start easier and 
> (in your case) idle smoother..  my guess is that whatever range your 
> trucks idling in'...  the fuel maps too lean ... 
> 
> Related to that ...  When's the last time you changed the fuel filter / 
> how's your rail pressure?  Maybe your fuel pump is also weak at colder 
> temps?  (just a thought!)
> 
> --Lee
> 
> 
> On 2/17/06, *David M. Ingebretsen* <dingebre at 3dphysics.net 
> <mailto:dingebre at 3dphysics.net>> wrote:
> 
>      
>     Hello all. Been awhile since I've lurked here.
>      
>     Setup: 1990 Jeep Wrangler, 4.2 liter inline 6, Originally a Howell
>     GM TBI conversion with 1227747 ECM, now happily modified and tuned.
>      
>     I've searched the archives, web, and FTP site and didn't find
>     anything which helped.
>      
>     The problem is this. When cold, (even using the Howell supplied bin
>     and their "cold" bin) at or near 0 C, the Jeep starts great, starts
>     idling fine but after some 15 to 20 seconds at idle, the RPM's drop
>     and it begins to idle very rough. When I drop it into gear (was
>     manual, now Chevy 700R4 auto, yes I added a pk/ntrl switch and have
>     verified its operation with WinALDL, and I am using the ECM to
>     control TCC lockup) and go, it runs fine. Smooth engine speed and
>     running. But soon after I stop (2 to 3 seconds) it starts to idle
>     rough, sputter, cough, on occasion die. If I poke the accelerator it
>     comes back and when I start, no problem. When fully warm, no
>     problem. When I start it at about 30 to 50 C, no problem. This was
>     the same with the manual.
>      
>     Question is what -exactly- controls the cold open loop -idle- AFR?
>     I've had no luck changing the O/L AFR table except to make it run
>     crappy before it goes into closed loop. I've adjusted the closed
>     loop temp threshold, cold and warm choke timer, choke table, cold
>     AFR v. MAP, time out decay mult. V. Cool. O/L idle AFR enable time
>     delay, O/L idle AFR time delay, closed loop timer warm threshold and
>     timer. I've looked at the Accel Pump v. Coolant but that doesn't
>     seem to be a problem because I can hit the throttle and it snaps
>     right in. I've searched the AZDZ hack to see if I could glean
>     anything, but didn't find anything useful.
>      
>     Like I said, it is fine when it starts warmish, but when it is dead
>     cold it struggles. So, my thoughts turn to the choke table, but I'm
>     not sure.
>      
>     Any ideas?
>      
>     Thanks all.
>      
>     David M. Ingebretsen M.S., M.E.
>     Collision Forensics & Engineering, Inc.
>     2469 East Fort Union Boulevard, Suite #114
>     Salt Lake City, UT 84121
>      
>     dingebre at 3dphysics.net <mailto:dingebre at 3dphysics.net>
>     Work: 801 733-5458
> 
>     	
>     	
> 
> 
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> 
> 
> 
> 
> 
> -- 
> Sincerely,
> 
> Lee M. Lemoine
> http://www.turbochargedsoul.com/ <http://www.turbochargedsoul.com/>
> '06 Subaru Legacy GT Limited - 250/250 AWD 5EAT
> '93 Chevy Cavalier Z24 Convertible - 300/330 FWD 5MT (Mclaren Turbo!)
> 
> 
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