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Sat Dec 28, 2019 7:04 am

I've got a 71 squareback I've had on the road now for almost 20 years. I really like the car and the fuel milage with the injection but it gets fussy trying to start it hot, always has. What happens is after driving to where the engine reaches temperature (180 oil temp) and I park for 8-15 min its very reluctant to start and when it does the rpms are low to the point of stalling. After a minute or two of running everything is fine. I have verified that:

compression is good (new rebuild)
valves and timing are on
good spark when hot
no vacuum leaks
new head temp sensor
fuel pressure is steady 31 psi, filter is clear

Ive tried both electric and mechanical aux air valves and electric seems to work the best. Ive read some threads, one mentioned the sensors getting hot with no air flow which I'm kind of leaning toward. It's like an oven under that deck. What's the solution guys?

Sat Dec 28, 2019 2:29 pm

There are several levels to this problem.
If its a rich hot start issue....it can be from fuel pressure dropping from injector bleed down (slightly leaking injectors). Put a gauge on the ring main temporarily. Next time its warmed up and you shut down for 15 minutes....before staryong take a look at the fuel pressure. If its below about 23 psi in a 30 minute period.....either your injectors are dripping into the manifold and you are getting a combination of poor fuel spray from low pressure (lean] or excessive richness from the fuel in the manifolds.
If the fuel pressure is under 20 psi....bump the key twice without starting to buzz the fuel pump....and check the pressure again. Then start it. If it starts well and idles normally.....its either the fuel pressure regulator leaking down or the check valve in the pump leaking down.

If it still starts and idles rough....and you get a rich smell.....its leakimg injectors.

This more common issue....excessively lean starting. The "main" issue is that your baseline fuel mixture is lean. Thats the main root cause.

The short term symptom is that when you let it sit for about 15 minutes.....the CHT or TS-2 sensor banks heat and drags the resistance down below 100 ohms. So.....on e you are fully warmed up and engine is running...pull over quickly and slip the colt meter + probe into the T-1 connector on the CHT and ground the negative to the case. It should read....probabpy 125-150 ohms.

Then shut down for 20 minutes. Before starting again check the CHT resistance. Typically it will bank heat drom the head with no airflow over it...and be down as low as 75 ohms.

Really.....the ECU does not havde any more leaning out capacity below wbout,150-200 ohms.....but,when the CHT drops too low.....it means its several minutes of maximum lean running before it has cooled enough from airflow to give any enrichment. In many cases a 150 ohm ballast resistor can fix this.

But really this is a symptom of being slightly too lean in the first place at your MPS adjustment. Ray

Sun Dec 29, 2019 6:14 am

Thank you so much for the insight into this system! The vw manual could only take me so far, I don't happen to have the VW1218 analyzer it calls for. Finally I have some diagnostic knowhow to solve an irritating ongoing problem. I've come close to going with carbs over the years. A couple of questions, where is the MPS adjustment, and what wattage resistor is the best to use for the sensor. I can't thank you enough for your time knowledge. James.

Sun Dec 29, 2019 2:23 pm

orau wrote: Thank you so much for the insight into this system! The vw manual could only take me so far, I don't happen to have the VW1218 analyzer it calls for. Finally I have some diagnostic knowhow to solve an irritating ongoing problem. I've come close to going with carbs over the years. A couple of questions, where is the MPS adjustment, and what wattage resistor is the best to use for the sensor. I can't thank you enough for your time knowledge. James.

Ok...a couple of things....and please do not take these as being pointy or picking on you. Its not meant that way.

You need to study up just a little on this system before getting too much deeper into it. :wink:

Ok...lest make sure how well things are dialed in. There must be "0" vacuum leaks no matter how trivial. Every single hose must be checked and clamped. The injection manifold runners must be checked, the connections at the head checked, injector seals should be new. Vacuum advance can checked for leaking and operation. The plate at the back of the center intake manifold should be checked for leaks.

You need an accurate gauge. Fuel pressure should be 28 psi. At a factory required 28 psi...every 1 psi of change is 3.57% of your total fuel dosage potential....so it matters. So if you are really at 31 psi....at WOT and several other part throttle settings you can be as much 10.71% too rich.

Along with this, the injectors are not reliable above 35 psi. Its a fixed pressure system with no 02 sensor...so the ECU ASSUMES that fuel pressure is stable and correct. For the moment...you are fine with the 31 psi you have..........but consider this:.....if you adjust your main fuel mixture and get it running "better".....and then later you buy a different or more accurate fuel gauge and find you are not at an exact 31 psi....and you re-adjust it....your fuel mixture will now be off again and the MPS will need to be adjusted again.

I cannot stress the importance enough of fuel pressure in this system.

Before doing anything.....go through your ignition. Pull the distributor...clean it, check it for play and proper function of the mechanical advance and oil the pivot weight fulcrum points. There are a lot of little details to cleaning and maintaining distributors. The more wear and slop you can take out of it the better....especially with the D-jet ignition distributors.

Take a look in this thread.

https://www.thesamba.com/vw/forum/viewtopic.php?t=686352

Also clean the trigger plate for the fuel injection in bottom of the distributor. Then check the resistance of each channel....and post it here.

The point of going through the ignition ...is that ANYTHING....that is variable or a negative on a D-jet system.....can cause a vacuum signature variation. When vacuum signature varies....that changes the load signal at the MPS (manifold pressure sensor) ....which is what sets your main fuel mixture baseline.

So...if anything from ignition to poor connections and poor voltage...affects the MPS vacuum signature even for a split second...it causes a rich or lean spike. That also affects vacuum signature....further causing changes to the vacuum signature.

Its a "cascade" reaction....and is on of the prime causes of what creates that "submarining" up and down idle that can be common.

So....also check to make sure the valve adjustment is SPOT ON. Ignition timing must be spot on.

Next....check and clean every one of the female connections inside of the fuel injection component connectors. Check, clean and tighten the three ground wire bundles under the center manifold on the case centerline. Check the voltage to the system when running. Its bare minimum is 9.5V...and it will run rich at that voltage. It needs to be 12 volts or above up to about 14.5 volts.
About 90% of all D-jet tuning issues are wiring harness related.

Only after all of this is done ....should you EVER...adjust the MPS. Its not HARD to adjust...but must be done exactly.....and typically requires an exhaust sniffer....though it can be done without one. To do it right....either with or without a sniffer...requires driving and adjusting and is a bit tedious.

When I adjust an MPS.....I use a fixed CHT ballast resistor with the actual CHT disconnected. The ECU only sees resistance to ground. I do this....so that minor up and down changes in temperature while I'm driving and pulling over to adjust....do not affect the adjustment by constantly changing teh fuel mixture. The ECU needs to see steady state temperature.

You do this by driving the car around a little. Do some steady state 40-50 mph driving and some highway. Then pull over quickly and check the CHT reading. Do this about three times. You want to know what the CHT drops down to when fully warmed up.

Lets say that is 125 ohms in medium weather like 60-70F. It will be lower in hot weather...and I like to use that usually.

So that resistance number is what you set the resistor to that you connect in place of the CHT for tuning.

Once you have the car warmed up and the resistor in place so the ECU thinks your CHT is at a steady driving speed temperature....you can stop and make an adjustment rich or lean to the MPS about 1/10th of a turn at a time. Re-adjust idle each time. Do about two adjustments and then let it cool off.
Drive a set circuit...some straights, hills, high and low speed. Pull over and turn it off an re-start and check starting function. Then turn it off and let it cool for a while...time it. Then restart...warm up...make an adjustment or two.

I do not subscribe to using an exhaust sniffer for this tuning....because with a stock exhaust there is no perfect or correct place to put the sniffer where it cannot sometimes be fooled.

It is more practical to adjust until teh car PERFORMS correctly...and then check the tail pipe emissions when stationary....to compare to factory numbers and make sure you are not too rich or lean.

You will need to buy a small variable resistor. I can give you a part number and you can buy online. This is the one I use. Its about 0-1000 ohms with 15 turns on the adjuster screw. Its under $2.

https://www.radioshack.com/products/1k-ohm-15-turn...0332255173

Before any of this you need to discover which type of MPS your system has. The diaphragm type actually has 3 adjustments inside......only two of which should you need to do.

Look into all of this and then we will go further into MPS adjustment. Ray