cylinder tin removal with engine in 412 variant

[andersonj]

Hi all,

Hope someone can assist me with an issue, I wanted to install a replacement CHT sensor and when I went through the hole to unbolt the sensor the head snapped off on the head, so I am trying to remove the cylinder head tin just on that side while the engine is still in my variant to get directly to the head to drill it out.
Question is, is it possible, I have already removed the manifold and fuel rail and all screws holding the piece in place, but it looks like I still need to remove the rear firewall tin in order to get it out. Has anyone done this before.


thanks,

[raygreenwood]

Yes...its a pain but can be done....yes...the firewall tin laps over the edge of the 3/4 side cylinder cover plate. I a pinch you can lower the engine a bit, remove the screws on that side of the firewall plate which also loosens the lower rear tin underneath which is attached to the pushrod tube cover plate.

If the left rear corner tin over the exhaust is removed and the chimney that goes to the left heat exchanger is removed...you should be able to twist the left side of the firewall tin out of the way enough to get the cylinder cover off.

In reality to keep from bending the firewall tin its easier to detach it completely. A lot of work. I have had to do it before.

Use a very small amount of aluminum or nickle anti-sieze on the new sensor....but not under the head of the sensor. You want to make sure there is perfect grounding no matter what. Ray

[andersonj]

thanks for the tip Ray, I will remove the firewall tin completely and see how it goes tomorrow.

[dawie]

Instead of using the cht sensor, i use the "manual choke" version (as was once mentioned by Ray). Cht sensor is still there, but disconnected.

Basically this is a variable resistor with a knob mounted on the dashboard. Before starting from cold, i turn this towards the rich side. Then use it as if it was the choke on a carburetter.

Main reason for this is to have peace of mind that i could do instant "fault finding" should a fault develop far from home. (Just richen the mixture while driving (or leaner while cold/just started), and see how engine reacts, easy way to see if mixture is way too rich or lean). Found this usefull when testing some spare components like manifold pressure sensors, ecu, etc.

[andersonj]

I think I remember reading that post, forgot about it though, sounds like something to look into, thanks Dawie

[raygreenwood]

I swear.......this forum is a conspiracy to get me to work on my car and my projects faster....under the auspices that ai am helping other people

I was just sketching this out again this last weekend.

Since weare talking about it....lets lay it out.

The reasons why I want to quit using the factory CHT sensor:

1. It has poor resolution. We know this is not an instant response tool. Thats not how the system uses it. The range is fairly broad and the system does not care about every 1 ohm increment. The system responds to fairly large jumps in resistance.

By this I mean you will not see significant differences at the tailpipe and how the engine runs....during the period of time between cold start and fully warmed up.....until the resistance on the CHT has changed quite a bit.
For example. ....when the engine is stone cold....depending on the ambient temps.....the CHT may read say......2800 ohms.

Start the engine and in a minute or two.....the ohm reading may be 2400-2500 ohms. You will probably see very little change in how the engine runs and what an 02 sensor in the tailpipe tells you. It may be 2100-2200,ohms before the first meaningful running changes come.

The next noticable change may come at 1800-1900 ohms.....the next at 1200-1500.....the next at 900 etc. Etc.

Typically the most noticable changes are coming in the 800-200 ohm ranges.

I keep using the word "may" because every system is different. How your baseline fuel mixture is set....based upon the MPS, fuel pressure, timing, wiring system resistance, system voltage, idle speed, AAR opening position and response speed and leven the load placed on the engine by the oil and how fast it warms up and shear thins.

A lot of the differences the more subtle changes in resistance make while warming up.....are not visible in how the engine runs.....but are noticble with a sensitive exhaust sniffer.

Because of the variances in other engine system tune level listed above....in many cars the factory CHT does not do a great job of keeping even or smooty running while warming up.

2. The CHT sensor ages over time. Its a thermistor mounted in epoxy. Over time with heat cycling.....its response range changes....and the time between "steps" of resistance drop become non-linear. Some of this is the overall design....as the cooling air that washes over it warms up.....its ability to shed heat its absorbing through the head become less.....and the rate of change speeds up.

3. The factory CHT has the ability to eventually drop to a resistance level that is actually too low. Some do this automatically even in cooler weather.....others may do this on the first harf driving burst....like accelerating onto the highway. The problem is that because the CHT surface area that is presented to the cooling air is so small. ...that the CHT once it heats up....will generally not be able to cool down to get back to normal fully warmed up resistance.

Also......this operation method.....is flawed. When you are accelerating hard.....you need enrichment. Most of this is handled by the load sensor......the MPS.....but at the same time....there should not be anything during these periods of hard acceleration that is actively trying to lean your system out....which is what happens when the resistance on this sensor drops.

4. In the real world.....the CHT needs to drop to a resistance level at fullly warmed up.....to give give correct idle baseline fuel mixture based upon the normal engine and cylinder head temps created by ambient conditions.

For instance......when its 75-100F outside the temperature of the intake air for the engine combined with the temperature of the cooling air flowing over the heads may result in a best running resistance of say.....100-150 ohms. But....during temp spikes that may drop to as low as 70 ohms and not rise again.

Likewise when its colder outside.....say.....between freezing and 60F.....when the enginecmay require a little more fuel enrichment.....at least for a while......you may find best running at say a resistance of 175-250 ohms.
Add to this....in cold weather. ....as everything gets fully warmed up and if say you are starting a long highway trip.....eventually the ohms may need to drop down fairly close to normal wark running resistance of about 100-125 ohms......but may not be able to because of freezing air flowing over the head of the sensor.

So.....in the winter you can get a combined issue of range versus timimg/speed control of the resistance drop.

You also have to remember that the CHT is a ground point. The system is measuring resistance to ground. The added insult is that the steel body of thr CHT can corrode or gall into the head. So it needs anti-sieze....which can affect the ground.

So.......a long time back I started experiments that used a variable resistor plugged into the CHT harness lead and grounded to the block......just to see what ranges of resistance the system responded to during warmup and while running.

I found as I already knew.....that the running of the system reaponded to fairly big jumps...and needed to be limited in its resistance drop in hot weather. ...and its resistance rise in cold weather. There are times when the resistance of the CHT can rise enough that combined with thin winter oil (lower load)....the input from the intake air temp sensor...wide open AAR....that starting can be slightly too rich when the CHT gets all the way up into the 3000 ohm plus range.

I found that MY engine.....could get away with a minimum of 5-6 resistance steps in the summer and 7-9 steps in the winter.

So....I spent a year playing with various ways to replace the CHT sensor with a variable resistor set instead of a thermistor like it has.

Some of the first I played with...that have some merit but I never got it worked out....was to use a small board with either a rheostat or variable resistor POT mounted...that was operated by a heated bi-metallic strip just like the AAR.
It would have worked....and one I tinkered with did....but required a trimmer POT connected inline with a cold/hot switch to cut it in or out for cold or hot weather operation.....whifh is something some of us already do ......in effect....by reaching into the engine bay at first 40F weather and either connecting or disconnecting the intake air temp sensor.

The best operating version ...or should I say fastest and easiet tuning.....was to run a single wire up to the drivers seat to a variable potentiometer so I could just dial it in on start up.

So......what I have been designing is a small board with a knob with detents....say....7 positions. There are 7 fixed resisters soldered on. Next to each resistor...inline....are a small Potentiometer with a narrow range adjustable with a jewelers screwdriver.

So the fixed resistors may be say.....300 ohm steps.....but the small Potentiometer allows me to make them say....325 or 340 and set them to stay that way. Some may be 200 ohms steps down in the middle and 100 down low and 300 up high.

There may be an all the way to the right, fixed super cold weater start or all the way to the left fixed hot weather position. Maybe 9 positions in all.

So...how to know what to set the knob to other than "seat of the pants" feel?.......well....as my car gets back on the road....I will be installing a small accurate CHT gauge....either at the right end of the dash near the windshield corner....or I may use the dash ashtray to have the gauge able to be folded away when not needed.
But.....the gist is that I ill use the factory CHT to first set up running ranges and make accurate note of the actual head and engine oil temps...and also seat of the pants running effect. I will then duplicate those running/warm up steps with the variable potentiometer to find out what resistances work best at what actual cylinder head temp is read on the gauge.

Then I will install those resistor packs on the board. Then I will print nice numbers or marks at the related temperature on the gauge face.....so during warm up period....I will know what knob position 1-9 the knob should be at.

Get in the car...turn the ignition to first position...gauge springs to life and notes coldest....turn to 8 or 9 and start. Follow the needle. When it dropw to 7, turn thee knob to resistor 7.
Within about 8 minutes generally. ....you can stop paying,attention to it. Adjustment is done and your lowest setting dialed in.

Just some thoughts. Ray

[andersonj]

Hi all,

so the replacement cht sensor is in and hooked up, I put in gas and replaced the battery and started cranking but it does not want to startup, sometimes its starts but idles badly then it dies, then it wont start again just cranks. I can hear the fuel pump starts.

The car did run before it was parked roughly 7 months ago.

I have a few FI parts spare, but I would like a rough idea of what could make it not start before I start testing and swapping.

thanks for any feedback

[raygreenwood]

[Lars S]

Agree to what Ray lists.
From my experience the injectors are the 1st to get more or less stuck when sitting for a longer period. Often they get going OK after some minutes of running but sometimes not. So checking them is a good idea.

/Lars S
_________________
Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold

[andersonj]

thanks guys for the tips, I will start by checking the fuel pump and all its surrounding components and move towards the injectors and distributor.

will keep you posted.

[andersonj]

So its been over a year since I updated this thread so time for an update:

Well last year Jan I decided to replace my old CHT temp sensor with a second spare I had and while removing the old one it snapped off in the head, so I had to remove the tin which was a lot of work. After I had installed the 2nd hand replacement sensor the 412 had issues where it would run for a couple seconds then cut out and would not start again.

I unfortunately did not have time to work on the car further so fast forward to a few weeks ago and I decided I needed to get the car up and running once and for all, I measured the resistance on the currently installed temp sensor and found it to be around 6.7kilo ohms at roughly 25degrees Celsius which is way over the specified 2.5k range when cold. I ordered brand new sensor and measured it and it measured 1.8k ohm, I temporarily connected it and grounded it and the car started and idled with no issues. I let the engine warm up a bit and then placed the sensor in some boiling water to get it around 800ohms connected it again and the 412 started and idled and rev'ed up a lot better.
So problem sorted, so next is permanently connect the sensor in the head and we good to go.

I am pretty glad its was just the sensor as I did a lot of testing on the injectors and trigger points and fuel pressure and found them all to big fine, and was starting to lose hope. I still have the old original 3 point fuel pump connected and was considering replacing it with the 2.1 vanagon pump as it does work, but will leave it as is for now.

[Lars S]

Glad you found it!
Yes the car wont run at all without the CHT (TS2) in difference to the air intake temp sensor (TS1) which do not have such big influence.
Possibly your sensor had a temperature sensitive failure...when it got warm its resistance went to open ciruit.

Lars S
_________________
Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold

[KTPhil]

If the main problem is that it goes too lean when hot, wouldn't adding, say, a 75 ohm resistor fix most of the issues? As you say, the difference between 2500 and 2575 ohms won't change anything, but the different between 75 and 150 when hot will eliminate the too-lean issue. I am thinking about this on my Type 3 for the same reason. Many have suggested ballasting the sensor like this. What rating and value of resistor works? And where to obtain now that Radio Shack is no more? I'd like to package it inside a plastic-sheathed connector like VW uses and make it plug-and-play.

[raygreenwood]

Also understand....that the TS-2...measures resistance to GROUND. The sensor needs to be grounded to the head. The head needs to have good ground to the case of course....and the engine needs to be well grounded at the transmission tail cone braid.

If the original problem was corrosion in the threads of the TS-2 that caused it to break off....its very possible there was enough corrosion to prevent proper grounding or at least give an augmented resistance reading.

Think about that for a second. If you have say....a perfect TS-2...which when tested in hot water reads 1800 ohms at room temperature.....and you install it in a very corroded bore in the head.....and you are reading the sensor with the positive probe of your VOM attached to single wire coming out of the top of the TS-2....and your negative VOM probe is on the engine case.....but the path to the TS-2...has to go through a layer of high resistance corrosion...very poor connection.....you can read a very high resistance to the TS-2.

This is why I test the"functionality" or the ability to change resistance accurately as temperature changes...... of the TS-2 with the heated water and VOM method...but then you have to spot check it when you install it to make sure its connection to ground is not skewing the reading.

I have only seen this to a failure level a handful of times....but it can happen.

KTPhil is correct...a 75-125 ohm resistor ballast in needed many times depending on your ambient conditions. Ray

[Lars S]

Yes adding of a ballast resistor to the TS2 can be wery helpful.

I mounted a 0-500 Ohms potentiometer as ballast resistor in the engine compartment (in fact I had it mounted in the coupe a while so I could tune and look at the AFR meter while diving).
When the best ballast value is found the pot can be locked or replaced by a fixed resistor with the same value as what the pot was set to.
There are no special high power data needed for the pot or resistor it can be of say 2Watt or larger type.

/Lars S

_________________
Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold

[raygreenwood]

Nice! I did the same tging with a variable ceramic resistor/potentiometer I bought,at,Radio Shack. Ray