Engine question, any ideas?

[soareyes]

For the last year or so I've noticed a hesitation/stumbling of my engine between about 3200 to 3800 rpm. This hesitation is only noticeable when I'm accelerating fairly hard through that rpm, but below and above that rpm everything seems fine. It is much more noticeable when the air temperature is warmer (like our Borrego blat last June) and I hardly notice it when its cold outside, just a hiccup. The engine is a mostly stock Zetec, fuel injected, controlled by a Pectel unit. Any suggestions on what to look at?

Thanks,
Stan

[JohnCh]

Stan,

Does it stumble when you tip in at that rpm, or just when you begin accelerating hard from much lower rpm? Also, when you write that it stumbles when you accelerate "fairly hard" does that mean heavy to full throttle, or just heavy throttle (i.e. it's fine at full throttle)?

Thanks,
John

[slomove]

Could be anything from cams to flaky MAP sensor to ignition hardware

But I would venture a guess that the main fueling map and/or the air & coolant temperature correction maps are a little off in that range. To say for sure you would need to get a wideband lambda probe or go to a dyno with that capability.

My wideband system is permanently installed but maybe somebody else here has a portable one.

Gert

[magnusfeuer]

The warm/cold weather part points to a rich running condition in that RPM/load-section. Cold weather means more oxygen per unit of air, which leans out the mixture. This can be compensated by the ECU through an Intake Air Temperature (IAT) modifier, but not all T2 maps use that feature. If the problem becomes worse at high altitude ( == lower air pressure == less oxygen), I'd say that you nailed the issue.

If this issue has gradually become worse during the last year or so, you should try to figure out why there is less air and/or more fuel going into your engine. It could be a failing mass airflow sensor (which you have if you run the stock Caterham intake system), reporting more air to the ECU than is actually flowing by it.

If you want to swing by me, we can take a look at the engine. I may have a MAF sensor somewhere.

/Magnus F.

[Woody Harris]

Stan,

Cut to the chase. Give me a call.

Woody

[soareyes]

Stan,

Does it stumble when you tip in at that rpm, or just when you begin accelerating hard from much lower rpm? Also, when you write that it stumbles when you accelerate "fairly hard" does that mean heavy to full throttle, or just heavy throttle (i.e. it's fine at full throttle)?

Thanks,
John

John, it will stumble either way (tip in at that rpm and accelerating from lower rpm), and either moderate to full throttle. If I just cruise at those rpms, steady-state jthrottle seems ok.

Could be anything from cams to flaky MAP sensor to ignition hardware.

Gert

Gert, are those spray MAP sensor cleaners safe to use? Thought it might be something quick and easy to try, but like you said, probably more to do with the mapping.

The warm/cold weather part points to a rich running condition in that RPM/load-section. Cold weather means more oxygen per unit of air, which leans out the mixture. This can be compensated by the ECU through an Intake Air Temperature (IAT) modifier, but not all T2 maps use that feature. If the problem becomes worse at high altitude ( == lower air pressure == less oxygen), I'd say that you nailed the issue.

If this issue has gradually become worse during the last year or so, you should try to figure out why there is less air and/or more fuel going into your engine. It could be a failing mass airflow sensor (which you have if you run the stock Caterham intake system), reporting more air to the ECU than is actually flowing by it.

If you want to swing by me, we can take a look at the engine. I may have a MAF sensor somewhere.

/Magnus F.

Magnus, do you still have the Pectec software on your laptop? I thought maybe you could look at the maps as I only have an Apple laptop (I know, I know!). Maybe I could come down your way one of these days. I first started noticing this over a year ago but I can't say it is getting worse, just annoying.

Now that I think about it, I had Woody's guys update my ecu to their newest map when I was there Oct 2005 with the USA2005 tour (they had an updated map from my sister car with the same specs). It was cool weather (and near sea level) from then on, so I may not have noticed the problem till next spring. As far as being related to elevation, I didn't notice the problem at all on our Maliblat (near sea level and cool), but did this last weekend (mountains and in the 70's). The worst was on the Borrego Blat when it was over 100 most of the day!

[soareyes]

Stan,

Cut to the chase. Give me a call.

Woody

Hey Woody, I just saw your post and will call shortly, thanks!
\
Stan

[magnusfeuer]

Stan,

I have the software and can probably borrow a wide band lambda probe for some real-time testing.

Call or mail me to set up a geek-date.

/Magnus F.

[moosetestbestanden]

My wideband system is permanently installed...

Right next to the portable refrigerator and the microwave oven. But I wonder, w/ that in there, where ya gonna fit the big screen TV?:D

[slomove]

Hey, it's a touring car...I need a little creature comfort :D

[soareyes]

Stan,

I have the software and can probably borrow a wide band lambda probe for some real-time testing.

Call or mail me to set up a geek-date.

/Magnus F.

Thanks Magnus, I'll get in touch soon!

Stan

[soareyes]

Well, after two years I finally found and fixed the problem!

It turns out that Birkin put a filter inside the tank on the fuel pickup tube on some of their cars from the 2002 to 2004 model years. Apparently the filter was too restrictive, and there is no way to get to it without cutting a hole in the tank and cutting the filter off the tube, which I did:



I just got back from the NorCal tour and I had no stumbling or hesitation in 1800 miles of driving. My fuel pump runs much quieter now too.

Special thanks to Damien, a Birkin owner in Australia, who knew about this problem and posted it on the Birkin owners list. The internet is wonderful!

Stan