Intake backfire, now car won't run. Help!

[LTDScott]

My M50NV swapped E30 LeMons car had been running perfectly fine up until now. Out of the blue this morning, when I started up my car it had a major backfire through the intake. The smoke was bad enough that I thought I something caught on fire. I shut it off and couldn't see anything amiss, so I fired it up again and it runs, but barely. It's backfiring/popping through the intake at idle, and will only run for a few seconds. When I crack open the throttle, it sounds fine, but the engine revs more than I'm actually opening the throttle. In other words, if I open it 1/4 way, it sounds like I'm going WOT for a second, then drops down. Here's a video:

http://youtu.be/OvK7a_RBfwM

Yesterday I swapped in new spark plugs and a new MAF boot (mainly because I had to change the brake master cylinder and I had no idea how old the boot was, so it was cheap insurance). Those are the only changes I had made recently, and after I did those things, the car fired up and ran perfectly. The car has fuel and fuel pressure (have a gauge at the fuel rail inlet), and when I pulled the plugs they were already fouled black and smelled like fuel. The car has always run a bit rich, but the car had literally only run for about 10 seconds yesterday, so I don't know why they'd be so soaked. I swapped in a spare MAF and DME separately and neither helped.

Intake backfires like that are typically caused by a lean condition (definitely not the case here) or incorrect timing. I hadn't done anything near the crank position sensor lately, but is it possible the sensor has crapped the bed all of a sudden?

Any help is greatly appreciated. I essentially have a week to get this fixed before a race.

[LTDScott]

Just to update my own thread, I'm not getting a tach signal at all while the engine is running. I just pulled the CPS off and the wiring pigtail to the connector is all chewed up. Can't believe I never saw that before!

I measured the resistance between pins 1 and 2 and it's 530 ohms. Bentley manual says it should be ~1280 ohms. So it's pretty clear the CPS is toast. I'm going to replace it tomorrow and hope that's the cause of the problem.

[e30rapidic]

check the big 1" hose connector in the middle/back of the intake, near where the throttle body is. I had a backfire that blew that out.

[LTDScott]

check the big 1" hose connector in the middle/back of the intake, near where the throttle body is. I had a backfire that blew that out.

Good catch! That got blown out and it looks like the clip that holds it into the intake manifold is broken. So I'll have to buy a new one :\

[LTDScott]

Here's what the wire to the CPS looked like.

[328ijunkie]

Bentley spec is wrong on the CPS btw.

[LTDScott]

Yup, I just came here to update the thread. The Bentley manual revision now states that ~540 ohms is the correct resistance reading for the CPS. That's exactly what mine was getting. So the CPS may not have been bad after all, but given how bad the harness is and the fact that the external shielding was broken, I'm replacing it anyway. We soldered up the old sensor, so at least we should now have a spare. I'm still concerned about why we're not getting a tach signal anymore.

I'm thinking (hoping) that the horrible running of the car after the initial backfire was due to the ICV hose popping out of the intake. Huge vacuum leaks aren't usually a good thing. I'm going to replace the CPS and the broken ICV hose elbow after work and cross my fingers. This is the last thing I need right now.

[LTDScott]

A new CPS and intake breather connector later, and she's back up and running. The tach not working seems to be an odd coincidence. I gave it a good whack and it started working again.

I think the initial backfire was caused by fuel pooling up in the engine. I think the injectors are leaking, so I'm going to get them inspected/fixed.

Thanks again e30rapidic for the suggestion on the intake hose. It would have taken me a while to find that on my own.

[Blaine0002]

Was there abnormally long crank when it started up? im assuming that would also be indicative to your injectors leaking (or fuel pump letting gas back in the tank)

[e30rapidic]

Thanks again e30rapidic for the suggestion on the intake hose. It would have taken me a while to find that on my own.

glad it helped!

[Mr.SWISS]

A new CPS and intake breather connector later, and she's back up and running. The tach not working seems to be an odd coincidence. I gave it a good whack and it started working again.

I think the initial backfire was caused by fuel pooling up in the engine. I think the injectors are leaking, so I'm going to get them inspected/fixed.

Thanks again e30rapidic for the suggestion on the intake hose. It would have taken me a while to find that on my own.

Gawd I<3E30's

[LTDScott]

Actually, this is an aftermarket tach. I'm still rocking the original Eta cluster, pretty much only for the gas gauge and charging warning light. So when I did the M50 swap I just bypassed the stock tach altogether.