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Help please, I'm stumped! Car was running fine, then suddenly, immediately after a moderate run-up to ~7000 rpm in 2nd gear, the engine absolutely will not idle. As soon as rpms drop below 1200 rpm, it immediately dies (no fire in the hole). It also dies under any minimal load conditions (maintenance throttle cruise). But give it more throttle, and it accelerates just fine. But the problem is bad enough that the car is undriveable.

I scanned the OBD-II port, and this is what I see: the MAF, TPS, rpm, front and rear O2 sensors all appear to be working fine. When the engine wants to stall, the O2 sensors go very lean. Hard to tell if its a cause or an affect, but it does cause the short term fuel trim to go full positive (+17%). No ETC (electronic throttle control) codes, no codes period.

Here are the things I've considered:

Intake air leak: I've looked (and looked, and looked), but I can't see anything amiss. This seems most likely to me, but where could it be hiding? Would have to be a pretty significant leak, not just a bit of seepage around a snug hose clamp. Also, wouldn't the engine tend to race at idle, due to the extra air, and the ECU dumping extra fuel to make up for the lean condition? Instead the rpm's will hang briefly at about 1200, then quickly drop and die.

Exhaust leak: an exhaust leak before (or at) the front O2 sensor would cause an incorrect lean reading at idle, which would make the ECU dump more fuel. I've removed and reattached the muffler, making sure the cat-to-muffler joint is tight, and it seems fine. I was esp. suspicious of this because I had recently changed the exhaust (stock cat pipe, 2bular 8x22 muffler). The front O2 sensor is also tight. And there is no indication at the tail pipe of an overly rich condition (due to the ECU dumping extra fuel), no gassy smell, no drip.

Water in the gas: I suppose it's possible, but the condition came on suddenly at about 2/3 full tank (after previous fill-up at my usual station). Also there is no visible white smoke from the tail pipe. I'd expect the engine to stumble under load too, wouldn't it?

Exhaust constriction: I would expect the O2 signal to read rich, rather than lean. Also the engine would resist revving, but it seems to rev fine.

Intake constriction: I wouldn't expect the O2 sensors to read lean, but maybe I'm just seeing lean when the fire goes out? Hard to tell, it happens fast, and the scan rate on the OBD-II scanner isn't all that quick.

Some variable cam problem: OK, I have no idea here. Say it "got stuck" on the second cams, how would the engine behave at lower rpm's? What can I check?

Fuel pump/fuel pressure/fuel injector problem: I doubt it's a clogged fuel injector, because all 4 cylinders seem to die at once. If it was low fuel pressure, I would think I'd see the opposite of what I see (idle OK, die under load), no? What sort of fuel pressure regulator does this engine have?

Is there something else I'm missing? Where else should I look? Any and all suggestions welcome. My last resort will be to flat-bed the car up to the Lotus dealer (about 40 miles), not too bad, but I'd really prefer to fix it myself.

Thanks in advance,
 

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I would of first thought an intake hose or filter came off, but sounds like you checked that out.
Some people have had their coils replaced, and sometimes down in the boot they look "rusty" Maybe give that a quick look?
 

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I would of first thought an intake hose or filter came off, but sounds like you checked that out.
Some people have had their coils replaced, and sometimes down in the boot they look "rusty" Maybe give that a quick look?
Thanks, good idea. I've focused on air and fuel, I've been taking spark for granted. Spark problems I've had in the past (other cars) usually show up at high load, high rpm, but I've been surprised before. I'll take a look.

Any other ideas...??
 

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shay2nak
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could be the SC vacuum hose if it's that bad. Those red thin tubes between the SC and firewall.
 

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could be the SC vacuum hose if it's that bad. Those red thin tubes between the SC and firewall.
Thanks for the suggestion, I thought I checked it, but it's not easy to see back there. I'll check it again when I get home tonight. I'm trying to get more familiar with this engine, what's the red vacuum hose for (connects what-to-what)?

Thanks again,
 

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shay2nak
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i'll throw up some pics real quick, but I got these from the cup box thread. I think one side of red line goes to throttle body and not sure where the other one goes.
 

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Discussion Starter #8
Try disconnecting the battery for a few minutes.
Michael
Thanks for the suggestion, I failed to mention it, but that was one of the first things I tried (after doing the OBD-II scan).

I still think an intake air leak is most likely, even though I haven't been able to find it yet. The fact that it started right after a brief WOT run, and now there is a severe lean condition at idle, sounds to me like something popped off under pressure (SC in boost), and is sucking air at idle. You'd think it would be easy to spot, wouldn't you? So why can't I find it?

Thanks again for the suggestions...
 

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Discussion Starter #9
i'll throw up some pics real quick, but I got these from the cup box thread. I think one side of red line goes to throttle body and not sure where the other one goes.
Thanks for pulling up those pics, that helps. -Ed
 

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Discussion Starter #11
Hmm, have you inspected the MAF and specifically the connector and the wires going into the connector?
Thanks for the suggestion, but yes I did. I also monitored the OBD-II port while the engine was running, and it seemed to be getting a valid MAF signal.

Last night I pulled the left side IC pipe to get a better look at the red vacuum hose (and all the branches), and it looks fine.

Any other suggestions?

TIA,
 

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shay2nak
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only thing i could say is to get a smoke machine and check intake and exhaust systems.
 

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I'm now having this issue with my '05 Elise. Did you ever find the resolution, AustinP? I would like to find out what's going on before I drive her again.
 

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Discussion Starter #14
The problem on my car was a leak at the exhaust joint where the downpipe joins the mid-pipe (before the cat and the front O2 sensor). It's the joint with the two spring-loaded bolts, and the conical fiber-metal "doughnut" gasket. The "doughnut" is easily damaged when doing exhaust work. The leak there was making the front O2 sensor read full lean at idle. It was OK at higher loads (higher exhaust flows) because the leak was relatively small.

-Ed
 
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