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Engine runs rough when cold


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I'm extremely frustrated with my 2001 Boxster S. It’s a long story but I’ll try to make it short.

I bought the car 2 years ago for a decent price knowing it had a few issues, thinking I would be able to fix them. One of these issues is the engine running very rough when cold. From a stand still, if I press on the accelerator more than half-way during the first 10 minutes of turning the car on, it will lose power and backfire. It does this even in neutral. When the engine is warm, all of these issues go away.
I took the car to several different shops, and none of them could fix it. They ran smoke tests to check for vacuum leaks, checked fuel pressure, replaced the MAF, temp sensor, AOS, spark plugs, ignition coils. One of the shops discovered it had hollow catalytic converters, and that the ECU might have been tampered with. I sent the unit to ecudoctors.com and they replaced it with a new one. After I installed the replacement, the car runs better than ever when the engine is warm, but still stalls when cold.
I don’t know what else to do. Could the hollow catalytic converters be causing this? Anything else I should check? I have spent what I would consider an excessive amount of time and money trying to fix this and I still can't get it to work properly. My excitement about having this car has almost disappeared now that it's turned into a money pit :(
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Any CEL and error codes? Have you tried disconnecting the MAF and drive around? Do you have Durametric or any OBDII scanners? I think the key is to find a shop that is willing to actaully diagnose it for you as opposed to just throwing parts at it.

Also, any mods on the intake? Cleaned the idle stabilizer?

Edited by Ahsai
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  • Moderators

I'm extremely frustrated with my 2001 Boxster S. It’s a long story but I’ll try to make it short.

I bought the car 2 years ago for a decent price knowing it had a few issues, thinking I would be able to fix them. One of these issues is the engine running very rough when cold. From a stand still, if I press on the accelerator more than half-way during the first 10 minutes of turning the car on, it will lose power and backfire. It does this even in neutral. When the engine is warm, all of these issues go away.
I took the car to several different shops, and none of them could fix it. They ran smoke tests to check for vacuum leaks, checked fuel pressure, replaced the MAF, temp sensor, AOS, spark plugs, ignition coils. One of the shops discovered it had hollow catalytic converters, and that the ECU might have been tampered with. I sent the unit to ecudoctors.com and they replaced it with a new one. After I installed the replacement, the car runs better than ever when the engine is warm, but still stalls when cold.
I don’t know what else to do. Could the hollow catalytic converters be causing this? Anything else I should check? I have spent what I would consider an excessive amount of time and money trying to fix this and I still can't get it to work properly. My excitement about having this car has almost disappeared now that it's turned into a money pit :(

If you have gutted cats and a tampered DME, there is probably no way the system is correctly controlling the fuel/air ratios, particularly if/when the secondary air injection system is running afer a cold start up. You are going to need to collect a lot of operational data on what the engine and sensors are doing during a cold start in order to figure out what you need to undo to get the car running correctly.

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Having done a lot of tamering with DMEs myself. I can tell you that in nearly all cases, unless you want a race car and drive at WOT all the time... the stock MAP and stock cats are best for daily driving (or some drivers events).

Typical reports of problems of ECU and intake/exhaust mods (and my own experience) is that you trade off low speed driveabilty for a few more percent of high speed/high RPM performance... not worth it IMO.

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Any CEL and error codes? Have you tried disconnecting the MAF and drive around? Do you have Durametric or any OBDII scanners? I think the key is to find a shop that is willing to actaully diagnose it for you as opposed to just throwing parts at it.Also, any mods on the intake? Cleaned the idle stabilizer?

No CEL nor error codes. Before I got the new ECU, I disconnected the MAF and the car ran a little better. It didn't backfire as much, but it felt a lot slower at higher RPM. I haven't tried this since I installed the replacement ECU last week. I have the Durametric cable. The parts I have thrown at it are based on the supposed diagnosis done by two of the shops. There are no mods on the intake.

Cleaning the idle stabilizer? Is that the same as cleaning the throttle body? If so, I have done that a couple of times.

If you have gutted cats and a tampered DME, there is probably no way the system is correctly controlling the fuel/air ratios, particularly if/when the secondary air injection system is running afer a cold start up. You are going to need to collect a lot of operational data on what the engine and sensors are doing during a cold start in order to figure out what you need to undo to get the car running correctly.

I told the ECU guys about the hollow cat issue so I think they programmed the replacement accordingly. According to them: "We did what we call a ROW conversion on your ECU where the 2ndary Air Injection is turned off and the 2post cat O2 sensors are ignored".I should have mentioned that I bought the car in Mexico. I'm not sure if it's a ROW version because the invoice says it was imported from the USA. I looked under the car, and I only saw 2 catalytic converters, instead of the 4 it's supposed to have. If I'm not mistaken, it only has what would be considered the secondary cats in the US version. Is this normal or did the previous owner do something shoddy? If this is normal, would it be a valid test to remove the catalytic converters and the remaining part of the exhaust to see if the issue persists?

Edited by skertchly
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Having done a lot of tamering with DMEs myself. I can tell you that in nearly all cases, unless you want a race car and drive at WOT all the time... the stock MAP and stock cats are best for daily driving (or some drivers events).

Typical reports of problems of ECU and intake/exhaust mods (and my own experience) is that you trade off low speed driveabilty for a few more percent of high speed/high RPM performance... not worth it IMO.

I agree completely, I don't want any performance mods on the car, I just want it to run like stock, but this has been the state of the car ever since I bought it 2 years ago and I've been trying to repair it ever since. I've spent more money on it than I should have, so I want to make sure that the cats are actually the problem before I throw even more parts at it.

From what you have seen in these other cases of exhaust mods, does the problem still happen after the engine is warm?

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Verified fuel pump volume?

Is this something a DIYer like myself can do? I have no idea how to test that, and I thought that verifying the fuel pressure would yield the same results.

Also, the fact that the car runs fine when the engine warms up made me think the fuel pump would not be the culprit. Do you think it could still behave differently depending on how long it has been on?

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Just like JFP said, someone needs to log a lot of parameters during cold start to see what the DME is doing or not doing. Best is to find a shop specialized in Porsche engine diagnostic. IMHO, there is absolutely no guarantee replacing the cats will fix the problem whatever answers you may get here because there are so many possibilities.

Since your problem in not intermittent, it should not be that difficult for a competent shop to diagnose the problem.

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If you want to try DIY, you can use durametric to log the following to start with during cold start idle:

Rpm

Coolant temp

IAT

MAF

TRA on both banks

FRA on both banks

Voltage of all 4 O2 sensors

Misfire counters

Close loop/open loop status

Ignition timing

Secondary air injection status

Keep logging the above for 5min then give a steady gas just enough to reproduce the symptoms for 10 seconds or so.

Repeat the above after car has warmed up and symptoms no longer exist.

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If memory serves me correctly, at cold start a stock USA ECU flash will do the following:

1. Run a combination of previuosly learned/pre-programmed fuel trims until the O2 sensors heat up. Once the 02 sensor heats up the DME uses the lambda readings to adjust fuel trims (and a few other inputs as well, MAF, IAT, etc.).

2. Run the secondary air injection system for a period of time (typically 90 seconds, but a slightly more complex algo is used).

3. Take mechancial timing readings from both cam sensors to alter ignition timing (does this all the time, and on RoW just uses 1 sensor and infers the position of the other).

A stock ROW ECU flash does not do items 2 and 3.

A mangled/hacked ECU, well... there's no telling what sort of monster you're trying to create/run, but if the idea is you don't want to run cats, personally I would just run a stock RoW flash. However, in California I would not be able to SMOG the car without cats or with an RoW flash.

Anyways, doom and gloom aside, assuming their flash is OK, I would be looking at your fuel trims to see how they might be contributing to cold start item #1.

Without the cat component you are also altering the dynamics of the pump, i.e. reducing backpressure, and that could also result in some low speed/low RPM strangeness.

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  • 2 weeks later...

Just a shot in the dark, but maybe the previous owner put some aftermarket fuel injectors in there. If they are injecting too much fuel, the engine could run very rich and cause your prolems. The O2 sensors don't catch the fuel mix problem until they are warm (see Logray's comment). Here's a how-to on the injectors:http://www.pelicanparts.com/techarticles/Boxster_Tech/25-FUEL-Injectors/25-FUEL-Injectors.htm

Edited by secretagent214
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