1991 7A Startup Idle Behavior

[cgharibo]

My 1991 7A coupe used to start such that the idle would increase to 1200 and then settle at about 900. The 1200 to 900 would obviously take longer when the car is cold.

What has now been happening is that the car doesnt start unless I press the throttle and do the same thing manually. If I dont depress the throttle, the car just cranks without starting.

I have cleaned my ISV and adjusted my idle speed without a resolution. My fuel pump is new and primes well, injectors are relatively new as well.

What are your thoughts?

[Bowie69]

Most likely a vacuum leak somewhere downstream of the MAF.

[Lago Blue]

These kind of snags are mind-bending aren't they!

I agree with Bowie's post above in so far as it is always good to do a physical survey when you open the hood too know the current state of things, however I'd also ask:

ISV:
- Have you cleaned it again, again & again?
- Can you hear / feel it functioning?

Codes:
- Do you have a functional Check Engine Light?
- Have you pulled codes?

G62:
- Might it be that your CTS or its' connection to the ECM is in some way faulty?

Possible explanation for current behaviour:
- Could it be that the preferred settings (for ignition, cold start enrichment & idle air) necessary for the current ambient cold temperature are now just off enough (due to the fault) that without some added throttle, it won't start?

Further Reading:
- Dave's related post for the blown cars (read especially about the crossed, worn & shorted wires issue):
https://forums.quattroworld.com/s4s6/msgs/20985.phtml

- A how to test the (related because it's a CIS VW) sensor post:
https://forums.vwvortex.com/showthre...erature-Sensor

- Steve's post re a crumbling ":AMP" connector:
https://www.s2forum.com/forum/member...29#post2000329

Photo:
Below is a page from a factory pub on the 7A's MPI system, note control unit's assumption (if CTS fault detected) in text:

CTS from the 7A MPI booklet 2019-03-11 at 22.52.35.png

Hth.

[steve briance]

"Possible explanation for current behaviour: - Could it be that the preferred settings (for ignition, cold start enrichment & idle air) necessary for the current ambient cold temperature are now just off enough (due to the fault) that without some added throttle, it won't start?"

There is some value in this comment. I have recent experience of some tuning on my 7A car and can echo that once your fault is identified and and fixed; it is well Worth getting the Idle mixture checked.
My car always passed its MOT test showing figures of 0.7-0.8% CO. On the face of it, this is really good. The problem was that the Ecu was ALWAYS having to make big adjustments to trim to this level and when cold, this can cause trouble if say, your Idle control valve isn't working as quickly as it should...

Part of the tuning process was to set the basic.mixture level so as to minimise inputs to the fuel and air control. My actual mixture setting was 0.3% CO - quite a long way short of the recommended 0.75%+/- 0.25%. When setting the mixture, the procedure is to remove any control the ecu may be providing and give you the true baseline point to start at.

Even if your problem turns out to be something unrelated, I think it is well worth doing now these cars are getting older.