Unique idle problem

[lolcantturn]

M42 has a little cammed V8 rumble at idle. I know theres a million threads that pertain to idle issues so I'll try and make it simple.

Symptoms:
V8 cammed idle (minus the V8 power heh)
Gets better when warmer.
Have to hold the throttle on initial cold start when it's cold outside.
When engine is warm, sometimes there is a long start.
On these long starts, often I can crank the engine over forever and the car won't start until I give it gas
Stomp test does not work.
Car actually idled pretty perfectly with the O2 unplugged, this is true when I have a new O2
Car idles the same with FPR unplugged.

Things replaced:
Mess under the intake has been done.
New ICV
New TPS
New Coolant Temp Sensor
New Cam Shaft Position Sensor
New O2
Tried different ECU's
Tried different AFM's
New Injectors

What I suspect:
FPR.
Check Valve needed at the fuel pump.
Harness issue
Crank Shaft Position Sensor


Losing my sanity. It's been like this for 70,000 miles :-\

[lolcantturn]

Also, I've performed a smoke test and nothing.

[lolcantturn]

Update: After many tedious attempts, I got the stomp test to work. I've owned many E30's that were all easy to stomp test, this one took quite a lot of tries to get to work.

Stomp test code: 1215 - AFM. I have tried different AFM's in the past and they have not done anything. Will check the wiring behind the AFM boot but I am sure it is good. AFM carbon tracks were also in good condition last time I checked.

My suspicion now is a clogged cat converter. This is the original cat converter on a car with 300,000 chassis miles.

Has anyone actually had an issue with a clogged cat directly effecting the idle of the car?

If it helps, my car produces an awful amount of backpressure, you can really feel the exhaust flow hitting your legs, but I'm not sure if that's normal on these M42's

The reason why I believe it wouldn't be a clogged cat is because when I disconnect my o2, after a while, the ECU will adjust and it will idle relatively perfectly. My o2 is brand new btw, and this would happen with an old or new o2.

[lolcantturn]

Here's the pressure my exhaust produces, pretty extravagant no? It also seems to be sucking the paper in a little. Note: The motor runs very strong with great compression across all cylinders. And I'm going to check the timing on the motor in two days, just to see if it's a little off.

[Bimmerbum318]

I don't know if this helps but my car did something similar on cold starting. Typically it would stall a few times till it was warmed up (pretty embarrassing stalling ten feet after fueling up). I did the mess under the hood, o2 and icv cleaning. Nothing worked. Then my starter quit and I took my intake off to find one of the coolant lines had a hole and that's what killed my starter but I also noticed a hole in the hose that goes to the fuel pressure regulator to the back of the throttle body. One simple hose caused many embarrassing moments.

[roguetoaster]

I don't know if this helps but my car did something similar on cold starting. Typically it would stall a few times till it was warmed up (pretty embarrassing stalling ten feet after fueling up). I did the mess under the hood, o2 and icv cleaning. Nothing worked. Then my starter quit and I took my intake off to find one of the coolant lines had a hole and that's what killed my starter but I also noticed a hole in the hose that goes to the fuel pressure regulator to the back of the throttle body. One simple hose caused many embarrassing moments.

Those were vacuum hoses, not coolant hoses for those reading in future.

[Bimmerbum318]

No it was a coolant hose. I know there is vacuum hoses but there are coolant hoses as well as you can see in the diagram. My hose was I believe 18 and in the diy write up they said to just turn the hose to where 11 goes onto the block. Needless to say it was to much stress for a 25 year old hose. Nevertheless all I was trying to say is how easily things get over looked and my problem happened to be the hose on the bottom left of the throttle body to the front of the fuel rail.