rough idle on 89 BBC

[unstable]

...I really dislike computers running the motor.

1989 C3500.
Recently changed rotor, cap, wires, fuel filter.

Gas tank was leaking, I wrecked the sending unit whilst dropping the tank and got rid of it for a holley blue (despite warnings about the failure rates on these pumps)...replaced all the fuel line. The regulator is pretty much set to wide open.

The thing idles like it's cycling...it seems like the RPM's climb a little and then start to die off...sometimes after the RPM's climb a little and it goes towards dying it stalls, other times it just keeps cycling. The RPM's don't go very high during this cycle, I'd guess it seems like it cycles between 700-1200 RPM's or so..

Any ideas? anything to look for?

 

[bizzo15]

sounds like a map sensor or vacuum leak to me. Have you checked for vacuum leaks yet?

 

[unstable]

I taped off a few ports on the front of the TBI...guess I could spray some ether around and see if there is something else that needs closed off.

 

[unstable]

I really hate computer crap!
I had the thought to disconnect the MAF and whatever the hell that other thing is that interfaces with the TBI and wow...now this thing is suddenly running like I would expect a BBC to run. I got the timing locked on and POW...just need to tighten a couple things down, put a little gas in her and we'll go for a cruise.

 

[bizzo15]

cool, good to hear you've got it running good

 

[unstable]

OK WHAT IS THIS STUPID THING?

Yeah I thought I had it running good too...at least she idles really good and I can sit in the driveway and rev the engine and it sounds good. Problem is, under load it farts out.

I have no idea what this thing in the picture is, but I suspect it is the culprit. It's on the passenger side of the TBI...picture is shot from passenger side...Mine is a green connector going into it...plastic tubing coming off of this stupid thing going to TBI.
As soon as I hook it back up, the truck runs like crap, she starts missing and farting and this upsets me.

 

[unstable]

Ok, I checked around online a little and I'm thinking that is the MAP sensor...measure manifold absolute pressure, so why does it plug into the back of the TBI? maybe the plastic tubing has a hole in it or something? SHOULD it plug into the back of the TBI or should it plug elsewhere?

 

[unstable]

replaced the MAP with a new one and it's still idling rough...unplug it and it idles fine...any other ideas?

 

[bizzo15]

you've checked all the hoses going to the map sensor for leaks?

 

[unstable]

I've only got one plastic line coming from the TBI going to the map, and I will admit that I didn't check it for leaks but I did stick my thumb over the hose/line and it felt like it was drawing strong vacuum.