Troubleshooting Poor Cold Start on a 3.2
 

This issue has been driving me a bit crazy and has me running in circles. Rather than continue to throw $100 at it each time I blindly replace a part I wanted to get a few other opinions before my next step.

Symptoms:

• Starts when cold and immediately dies
• When running it has short power cuts during hard acceleration
• Wants to die when taking off from first gear, need to give it plenty of throttle to get going or it will die
• The idle is just over 1100k RPMs and does tend to "bounce"
• Occasionally, when I go to turn the car over the key feels manually locked out of the start position. After taking the key out and trying again it works after a few times.

Other Facts:

• If I give it a little gas while starting it will eventually find an idle and warm up.
• While its idling, and still cold, if I even look at the throttle to soon it dies.
• It warm starts just fine.
• Separately, I can jump the CHT at the female connector and it fires right up and idles at 800 RPMs. It does run a little rich in this situation as indicated by small amounts of white smoke.

What I've done so far:

• I purchased this motor rebuilt by a previous owner, many items were replaced with new, specifically the fuel injectors had been cleaned and the fuel lines refreshed.
• All new Plugs, Cap and Rotor, fuel filter(No Change)
• Cleaned and bench tested the ICV (No Change)
• Replaced the Idle Control Microswitch (No Change)
• Replaced the Cylinder Head Temp Sensor (No Change)
• Swapped in a known working Coil (No Change)
• Fixed a minor vacuum leak at back of Throttle Body (No Change)

... and thats where I'm at today. I have a friend who is going to let me plug in my DME into their working car to rule that out, but I'm not sure where to look next.

I read through a bunch of threads on the various subjects. Based on those I am looking into a few other items, but I'm not convinced they are the issue.

DME Relay - I get power to the fuel pump. My understanding is it wouldn't even start if this was the issue, but I may be wrong.

Crank Position Sensors - They are gapped correctly. I do have an over the transmission oil line from ER that runs near them, not sure if that could cause problems. My understanding with these is also that the car wouldnt start at all if they had failed.

Throttle Position Sensor - This would explain the hesitation under power, but I dont believe it would impact the cold start and idle.

Anyhow, I think thats everything I've done to this point. I'd love to hear your thoughts or just some words of encouragement so I don't give up. :)

There are a lot of threads on 3.2 starting and idle issues, so please continue to search and read through those to gain as much info as you can.

my initial thoughts:

If it were mine, Id start by smoke testing the intake system to make sure there are no additional vacuum leaks. There are a lot of opportunities for leaks on these engines including the multi layer stack of intake manifold gaskets. smoke testers do a fantastic job, so I would not skip this in an effort to improve your chances of finding any leaks.

Poll and inspect the connection of the vacuum lines at the fuel pressure and damper locations. if you have any wetness or smell of fuel at these vacuum connections it is a sign of diaphragm failure of the damper or regulator. replace as necessary.

replace fuel filter

make certain that connector for CTH is secure at left manifold.

verify condition of all fuses, clean and replace as necessary

Thanks for the suggestions, I had been looking into getting a smoke tester, probably a good thing to have either way.

I forgot, but the fuel filter was also replaced, I added it to the list. I inspected the fuses and everything was good.

That typically indicates a bad temp sensor causing an over-rich condition. You also may have a bad fuel pressure regulator or a bad fuel damper.

How old's your fuel? White smoke can indicate water in your fuel, and modern petrol can be hydroscopic.

What colour are your plugs? That might shed some light on the mixture.

Does the idle change in response to removing the oil cap? It will do if you have good vacuum.

Check the resistance of your sensors at the ecu if you can, as it confirms continuity of the wire all the way back.

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The fuel is fresh.

There are new plugs in, I’ll have to pop one off and take a look.

When I pop the oil cap nothing happens. I figured the same as you, vacuum leak. But when I was reading threads on here it suggested the DME would just compensate for that and nothing would happen. Is that not true?

I’ll have to check resistance at the ECU to confirm sensors are functional and there isn’t a separate wiring issue.

Thanks for the ideas!

The engine is running, right? Then the 2 sensor (RPM & TDC) are OK!
My previous post (4) indicated that your temp sensor is bad. Replace that!

Don't become overloaded.

I did replace the Cylinder Head Temp Sensor. The previous one was also new when the motor was rebuilt. I had even tested them, both come back in the expected range.

If there's not a noticeable change, at least momentarily, when you remove the oil cap then you probably have a vacuum leak.

This probably won't be your whole problem because it usually just causes a rough idle rather than anything worse.

Unless you have a lot of unmetered air going in, which could cause a significant lean running problem.

Check the easy to access spots first. If you bend the hoses around the oil cap, there's sometimes hidden cracks at the elbows. Check all hoses are connected. The hoses to the two fuel pressure controllers at the backs of the fuel rails, and a key spot is the three on the back of the throttle body. If something comes off back there it often goes unnoticed. You'll need a mirror or reacharound and take a photo with your phone.

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I’ve been troubleshooting without keeping a good enough list. I forgot to mention that all of these lines I attempted to highlight in red did not come with the motor. Everything is capped, and I don’t think it’s required to run the car.

I will go through and double check the other lines.

http://forums.pelicanparts.com/uploa...1711087795.jpg

Why do you continue to tell the OP he has a vacuum leak? It's too rich. That's NOT a vacuum leak. You're confusing the OP! Read the first post again;

A 911 3.2 is the easiest pre-2000 Porsche to troubleshoot. Hopefully, the AFM spring hasn't been "played" with, that can easily cause a rich condition and a poor idle.

Before wasting more time and money, the OP needs to follow what he mentioned in post #1 now;

The OP needs to eliminate the DME ECM and Mickey Mouse "performance" chips. They can cause many problems.

Please, no more guessing.

I didn't. What I said is:

If there's not a noticeable change, at least momentarily, when you remove the oil cap then you probably have a vacuum leak.

And that's a 100% scientifically verifiable true statement.

The PROBABLY I put there accounts for rare exceptions such as e.g. if the whole vacuum system is blocked off.

No guessing involved. Oh hang on looky here, what's this sitting here? The 1988 911 DME test plan.

The fail you made here is to ASSUME only one fault. Here's the kicker: there can be TWO defects in a system, or sometimes even more - who knew!! The clue was there for you, to be fair, where I wrote:

This probably won't be your whole problem because it usually just causes a rough idle rather than anything worse.

I beg to differ because look what happened - my scientific approach of gathering further evidence before jumping to conclusions yielded the additional data that half the vacuum system is shut off.


Ah I see, you have a chip on your shoulder. Maybe go again without the attitude and you'll do better next time.

I'd check the vacuum is getting to the fuel regulator and stabiliser.

If not, sadly, this complicates the situation quite considerably. The way the Bosch DME system works, is the vacuum hoses connected to the fuel pressure regulator and stabiliser, maintain the fuel at a constant pressure RELATIVE TO MANIFOLD PRESSURE. Without those connected, your fuel delivery will go leaner at open throttle and richer at closed throttle, and the effect will be more pronounced at higher revs.

Left alone, this will mean you're rich at idle, you're still rich at half throttle, but not so much, and when you open to full throttle, the mixture will correct itself. But with the rich idle, there could be unburnt fuel hanging around or sooted up plugs, messing up performance with the engine at other states.

Additionally, it's likely somebody could adjust the mixture to compensate at idle, resulting in a lean running situation at open throttle.

My recommendation would be to connect the vacuum system back up and go from there. It looks complicated and daunting, but it's not too bad if you have another car to copy. Many local owners would be willing to accommodate that, I'm sure.

If the hoses are missing and you don't want to go to a new system such as ITB's or carbs, then my recommendation would be to find a shop which backdates Carrera's or adds ITB's, and they will have a load of Carrera manifold systems going spare. I know Rennsport in the UK have six or seven listed on eBay, but I assume you need one in the USA.

If you wish to verify the vacuum system functioning then fit a fuel pressure meter to the port at the front left fuel rail. Fuel pressure should drop with manifold vacuum. If that's not happening then this is one of your problems.

Finally, a common failure on these at this age is the diaphragm of one of the fuel pressure regulator/stabilisers starts to leak fuel. After running the engine for a while, pop the vacuum hose off each regulator and check for traces of fuel in it. If the regulator is leaking fuel in the pipe, the regulator needs replacing.

Good luck!

P.S. Just going on instinct, my view is this:

Your fuel pressure regulators don't have vacuum because removal of those pipes has opened something to the air.
Consequently you are rich at idle
Given the removed hoses, most likely this is the oil tank.
Solution: seal any openings in the oil tank / seal open hoses coming from it.

Second likelihood is vacuum failure in the hoses leading to the fuel regulators.
Verify this by checking the regulation of fuel pressure relative to manifold pressure.

Third likelihood is fuel leaking into the vacuum lines from the diaphragms of the regulators. (Easily checked)

Finally, once all these are ruled out, you may still have other disregulation. Those pipes were probably removed because there was another problem pre-existing. And other maladjustments may have been made to compensate.

@mysocal911 @RobFrost

I appreciate both of your insights, all the feedback is helpful. It is entirely possible this could be more than one problem. The motor did sit in storage for a year and a half before going into the car. Even though the motor was refreshed many of the parts could have become stuck or brittle from the lack of use.

I had this problem once. The thing that saved me was Air Fuel Ratio meter. That told me the engine was running rich and the computer was trying to compensate but unable to.

The solution for me was to open up the Air Flow Meter, and found that the previous builder has changed the factory setting to compensate for a different injectors / computer. He then removed said modification to sell the car and never reversed the AFM changes. Put back the factory setting, and the car ran solid after that.

That maybe a problem for you, but you need to NOT guess and just get the Air Fuel Ratio meter.

You may have a sensor on your exhaust already. If so can put a multimeter on that to get your AFR. It comes up through a round hole in the tinware to the left of the engine.

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Yes, this was mentioned in post #11. Hopefully, the wrong injectors weren't used, i.e. does occur since the original ones are not easy to find.

How did you find this leak ?

I took the throttle body off and inspected the lines. One had a clear tear in it.

Just pulled off the AFM and bench tested it with a 9v battery. Starting out I got a reading of .28v. As I opened the barn door the volts steadily increased until it was fully open and rear 5v.

From the articles I found I would expect this to read 9v at full open. I can see two groves on the connection track. Makes me think I should adjust the arm to get a better connection.

How else can I tell if the AFM had other adjustments made?

Yes, measuring voltage is used, but it's the spring tension which determines the fuel enrichment versus air flow.
To lean the mixture, one just increases the spring tension for smooth engine RPM acceleration.

little help for youhttp://forums.pelicanparts.com/uploa...1711150352.jpg

You could try pumping air into one of the ports, see if it holds pressure and hunt for leaks with a spray bottle.

Please explain why you suggest he needs to search for air leaks, when he has a rich mixture?
When air leaks are the problem, the mixture becomes lean!

The OP's 1st post states the mixture is TOO rich and has to bypass the temp sensor to allow improved starting.

OP, have you measured the AFR as recommended prior ?

I think you are jumping the gun by skipping the measuring step and going straight to thinking about adjusting this precision instrument below.

Resist the "encouragement" to tinker. In any case, without a means of measuring the effects, you are flying blind.

Hope the brain trusts are working on a modern replacement for this 40 yr old electromechanical marvel.

The nice part of having an AFM is that one can easily move the AFM wiper, without adjusting the spring, and determine whether the AFR is too rich/lean based on how the engine reacts.

Redo the vacuum check. It's quick, immediate results, no tools required, and highest frequency cause of problems on these cars.

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It still won't explain why the engine is running TOO rich!

It only starts running rich when I jumped the CHT sensor. It’s a bit on the lean side when the sensor is plugged in.

Did you measure it with a AFR, or with your calibrated nose?

Haha, just the sniff test for me. The mechanic at the shop that put the motor in said it is running lean. He also suggested is was just the idle control switch, or the need to clean the ICV. So who knows.

They had the whole intake manifold off to replace some leaking fuel lines. They said they inspected and replaced the gaskets. I’m sure they didn’t vacuum test it.

You, like some, failed to read/understand the OP's first post;

This implies that the mixture was too rich with the temp sensor connected, and that by jumping it (leaning the mixture), it ran normally.
It doesn't get any easier than that! Tests like that are why the 911 3.2 is very easy to troubleshoot.

Hey guys, get with it, this isn't a CIS engine!!!!!!

When you say you jumped the CHT sensor, you mean you shorted (with a paper clip or whatever) the sensor connector terminals on the harness side, right? Not on the sensor side (this wouldn't make sense).

In that case I wonder why this makes the engine run richer, since shorting the CHT sensor input to the ECU simulates a warm engine. The CHT sensor is a NTC. The lower the resistance, the higher the temp. Shouldn't it be the opposite?

He assumes this!

Date, post #30: "It only starts running rich when I jumped the CHT sensor. It’s a bit on the lean side when the sensor is plugged in."

Who knows what kind of DME ECM chip is installed? It's total confusion at this point. No more guessing.

As I said up-thread, time for another DME ECM!

There're too many unknowns at this point for any productive troubleshooting;

1. DME ECM chip type
2. Injectors being used
3. Fuel pressure
4. FP/Damper leaks into intake via vacuum lines
5. AFM incorrect spring tension
6. Throttle enrichment switch shorted to ground

I took the ECU out to hopefully test it in a friends car soon. Figured I would open it up to see what chip is in it. Clearly it had been opened before. Chip has a sticker on it, PAE i8. I did a search and couldn’t find a reference for that.

I had also opened up the AFM and noticed the spring was moved one notch towards the lean side.

http://forums.pelicanparts.com/uploa...1711216010.jpg

I agree, this is much more complicated than I originally hoped, or had described to me.

I posted the chip and AMF spring settings.

To my understanding these are OEM injectors that had been cleaned and refreshed.

It does seem like it might be a fuel pressure issue. I took the vacuum lines going to the FP damper and regulator off, they weren’t wet, but they smelled of fuel.