cold start drivability issue

[dabaron]

the patient:
1991 2.1 automatic - engine was running ok* in the tin top before i swapped into the Westy.

issue:
cold start engine under load will bog down then pulse to life, then bog down. when bogged down there is a "flappy" sound to the exhaust and a random small backfire (more of a pop than a bang). almost seems like it's running on 3 cylinders, but at idle and rev'd up to 4k with no load it sounds good.

this will repeat until the engine is warmed up - usually the first line on the temp gauge. once warm, zero issues, drives great, lots of smiles. if the engine cools down "enough" the issue will be present upon restart. if still warm, no issue.

items checked:
TPS - within spec
Temp2 and wiring to ECU - within spec - from tin top
O2 sensor and wiring (no shorts to ground) - within spec - from tin top
AFM/AFM temp/wiring to ECU - within spec - from tin top
good clean grounds on engine, chassis, tranny
swapped ECU from tin top
pulled Idle Stab Module, opened and inspected, looks brand new
ICV buzzes as expected - from tin top
coil - known working - from tin top
power steering fully racked, idle drops and engine almost stalls (bad wiring or switch)
a/c belt not connected, system off at dash

items not checked:
plug wires - no issue before swap
injectors - no issue before swap but Westy fuel system was the nasty
fuel pressure regulator - from tin top
fuel pump (have replacement on hand)
Idle Stab Module wiring
power steering switch and wiring


since it did not do this in the tin top, i suspect an issue with the wiring in the Westy. since the usual suspects all test good, that leaves the idle wiring.

i believe the idle stab module only does it's magic until the coolant temp is above X temp and then it hands off control to the ECU, is that correct? if so, that seems to make sense and my stink eye toward the idle system is warranted...

i want to better understand the interaction of the idle control system and logic, but it's not in the Bentley nor the Protraining manual. have i turned past it in one of those resources?


*after chasing down several vac leaks in the tin top , it still had a random skip after first 10-15 seconds at idle, cold or hot engine temp. the issue followed over to the Westy.
_________________
1991 Vanagon GL Camper
i had no idea i wanted to be a mechanic

[mikemtnbike]

I had similar symptoms and found backing off my valve lash to help per ken wilford’s post in this thread. Can’t hurt to check it.

https://www.thesamba.com/vw/forum/viewtopic.php?p=6355789&highlight=#6355789
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1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual

[dabaron]

[zoti]

"power steering fully racked, idle drops and engine almost stalls (bad wiring or switch)"

This tells you your idle control unit is not activating the valve when it should. Does idle drop as well when you turn the AC if you have one?

You can short the wires at the power steering switch to see if it's a bad switch or idle control unit. Those units go bad even if they don't look bad. If you have another unit, you can swap it and try.

I'm fighting a cold start issue myself right now but no solution so far.

[dabaron]

[DanHoug]

exhaust sound abnormality and popping in the intake is unrelated to the idle system, which merely adds bypass air around the throttle plate. that said, block off the both sides of the ICV system... at the manifold and in the intake boot. this will allow you to focus on other issues. mines runs great without the ICV system.

you COULD have a sticking intake valve so put a quart of ATF in your gas and drive around a bunch. if you want to get more aggressive, remove the injectors and squirt a bunch of brake fluid in the injector hole. had a nasty sticking intake valve on a small engine and brake fluid was the only solvent i found that would free it up, and i threw everything at it in a spray can. if you had some GM Top End Cleaner that will also work. but with the design of the intake manifold don't suck it in thru a vacuum port like you might on a 'normal' engine.

so. bypass the ICV system (to completely eliminate vacuum leaks and extra data) and get rid of the popping would be my first order of business.
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[zoti]

[DanHoug]

the actual starting of course is unaffected. in temps below 10F i might have to keep my foot on the throttle for less than a minute, more like 30 seconds. i truly don't miss it.
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[zoti]

[DanHoug]

No. You just keep the rpm up around 1200 manually with the throttle. Works fine down to -30F
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[zoti]

[4Gears4Tires]

Well it won't get warm enough if it keeps dying.

So keep your foot on the pedal.
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'87 Syncro Ferric Oxyhydroxide Superleggera Edition
'85 Westy Sciuridae Domus Edition

[Wildthings]

I would try some FI cleaner and/or the ATF that DanHoug mentions. You could have an injector that is sticky until it warms a bit.

As far as the valves, they like clean thin oil, which in my book means synthetics with a high viscosity index. I am presently running Mobile 1 0w40 European Car oil in my 83 1/2 with a used 2.1L engine in it. It sat for 6 months over the summer and the valves were absolutely quite when I fired it up a month ago. The valves are adjusted to 2 turns and never give me any trouble at all.

[dabaron]

what confuses me about all of this is the consistency. the engine will bog down EVERY cold start until the engine reaches X temp. the problem does not exist when starting the engine above X temp. i have not figured out what X temp is, i have no temp meter.

the idle speed is around 900 with a slight bounce, regardless of engine temp. this stumble in the idle has been reduced over time as i discovered vac leaks; brake booster, cracked intake boots, deformed injector seal.

this might have nothing to do with this, but i have noticed when i remove my foot from the accelerator while driving after warm up, the rpm's will start to drop and just below 2000 it will come back up to 2000 and hang there briefly and then drop back down. this is a recently rebuilt automatic with a peloquin. the throttle is not sticking, lifting the pedal has no effect on this behavior. it does not do it consistently, but when it does, it's noticeable.

vanagons.
_________________
1991 Vanagon GL Camper
i had no idea i wanted to be a mechanic

[4Gears4Tires]

Issues like this can be the result of compound problems. Definitely finish your list of items not checked. I would replace the plugs, wires, coil, and clean your injectors.

The way I clean injectors is by using carb cleaner, a valve stem, a cap, power source, and some alligator clip cables.
-Drill a small hole in the valve stem cap.
-Super glue the red carb cleaner tube into it.
-Take the shrader valve out of the valve stem.
-Put the valve stem over the injector.
-Put the cap on the valve stem+injector.
-Put the tube and everything else into the carb cleaner, stack some random crap to place the injector on.
-Connect one power lead to the injector and power source. Connect the 2nd to the power source but not the injector.
-Carefully and with proper timing, spritz the carb cleaner and bump the 2nd power lead at the same time. Never hold power to the injector for very long as these things are designed to fire really fast, much faster than you can do, but quick tappy taps are fine.
-Enjoy the spray pattern going from a sad stream to a full fountain.

Note: If you spritz the carb cleaner and don't bump the 2nd power, the valve stem will shoot off and carb cleaner will go everywhere. Wear proper PPE. I have done at least 40 injectors like this and they have all seen an improvement.
2nd note: Is this as good as getting your injectors flow matched? Or buying a set of flow matched injectors? No, don't be silly. But it's nearly free and only takes 15 minutes.
_________________
'87 Syncro Ferric Oxyhydroxide Superleggera Edition
'85 Westy Sciuridae Domus Edition

[dabaron]

djkeev came by to look at his tin top and i made him suffer looking the Westy... the miss at idle is present off idle at various rpms.

this weekend i'll do the carb cleaner routine to look for vac leaks and see if i find any smoking guns. i'm also going to send a set of injectors out for the needful on those.
_________________
1991 Vanagon GL Camper
i had no idea i wanted to be a mechanic

[onslowb]

Sounds like a vacuum leak to me. I would try blocking off the idle valve and turning the air screw in the AFM all the way in counting as you do so you can put it back to the current setting. You may also have to adjust the idle screw on the throttle body. If there is a small intake leak somewhere the engine will run too lean during warm up before it kicks in to closed loop. This is what is causing your stumble and lean misfires. Spray intake manifold, intake boots/hose things, and throttle body shaft w/ carb cleaner or something to check for leaks. Also block off brake booster line. Could be leak there and that wasn't changed from tintop.
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1987 Weekender 2.4GW manual
1970 Vw Bug SP 1600

[djkeev]

It is an odd miss.
It was not there when the motor was in the tintop. It only showed up once installed in the camper.

All the obvious sources have been checked, all electronics swapped out with good spares.
New throttle body.
The injectors are new but aftermarket ones I gave him.

I raised the rpm's up and while the miss became less severe, it was there consistently.

Engine temperature doesn't affect it.

Bad spark plug wire?
Bad spark plug?
Bad injector?
Small vacuum leak but honestly it seems to be running rich if anything and lets out a random backfire.

I didn't check the timing.

Dave
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Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[dabaron]

what should the voltage be on pin 6/50 (R/BK wire)? should that voltage be present at key on or only when the engine is running?

_________________
1991 Vanagon GL Camper
i had no idea i wanted to be a mechanic

[crazyvwvanman]

The /50 in the pin designation means it comes from Circuit 50 which is the starter motor connection to the ignition/starter switch.

This is meant to tell the ECU and the ICU that you are attempting to start the engine.

Mark