78 280z idle problem

[Jack Pearcy]

Hey guys I’m new to this website but I have read a ton of stuff on here and it’s been a great help so that’s why I’ve joined. Anyways my 78 280z has been running fine for a while, I have all new fuel injectors, fuel pump, and afm. My fuel pump relay was bad so I wired up a switch to the wires where the fuel pump relay normally connects to and it’s worked fine. Recently however it has been idling horribly at around 500 rpm, but when I flip the fuel pump switch to off it jumps right back up to 1000rpm and idles fine until it dies from fuel starvation. It will also occasionally die out in addition to not accelerating sometimes. I’ll be driving and suddenly the power will not exactly cut out but not change if that makes sense. It’ll hold steady but stop reving and accelerating. It also blows darkish white smoke out of the exhaust when I rev it.

I know it’s a lot of info but I can’t figure this out if anyone has had a similar problem or a solution please let me know!

[Patcon]

Have you done a compression test?

I think that's where I would start

[Mark Maras]

 Does the darkish white smoke dissipate or does it hang in the air? If it dissipates, it's likely steam. Oil smoke (bluish) tends to hang in the air for a while. That's why Charles recommends a compression test. You'll have more info when you pull the plugs.

[jonbill]

Could be running rich from too much fuel pressure. Idle going up to 1000 when you turn the pump off supports this and the dark exhaust could be fuel not oil. Is the fuel pump & fpr giving the right fuel pressure?

[Zed Head]

Check the vacuum hose to the FPR for raw fuel.  Could be a hole in the diaphragm.

[Jack Pearcy]

18 hours ago, jonbill said:

Could be running rich from too much fuel pressure. Idle going up to 1000 when you turn the pump off supports this and the dark exhaust could be fuel not oil. Is the fuel pump & fpr giving the right fuel pressure?

That’s what I was thinking, I’ve heard I can adjust this with a screw on the back of the AFM

[Jack Pearcy]

12 hours ago, Zed Head said:

Check the vacuum hose to the FPR for raw fuel.  Could be a hole in the diaphragm.

I’ve been looking into the possibility of vacuum leaks since the car had been sitting for 13 years before I bought it and I replaced every hose that carries liquid but never touched any vacuum lines

[Jack Pearcy]

On 6/15/2019 at 8:56 PM, Mark Maras said:

 Does the darkish white smoke dissipate or does it hang in the air? If it dissipates, it's likely steam. Oil smoke (bluish) tends to hang in the air for a while. That's why Charles recommends a compression test. You'll have more info when you pull the plugs.

Smoke hangs around for a bit, doesn’t dissipate quickly like steam, I haven’t done a compression test yet but I’ll look into it

[Jack Pearcy]

On 6/15/2019 at 5:17 PM, Patcon said:

Have you done a compression test?

I think that's where I would start

Okay thanks!

[Zed Head]

34 minutes ago, Jack Pearcy said:

I’ve been looking into the possibility of vacuum leaks since the car had been sitting for 13 years before I bought it and I replaced every hose that carries liquid but never touched any vacuum lines

It wouldn't be a vacuum leak, but you'd be sucking fuel straight through the FPR.  Vacuum on one side of the diaphragm, fuel on the other.  It's a flexible fabric that gets old and cracks.  A small crack can let a lot of unmetered fuel into the intake manifold.  Not uncommon.

[Jack Pearcy]

22 hours ago, Zed Head said:

It wouldn't be a vacuum leak, but you'd be sucking fuel straight through the FPR.  Vacuum on one side of the diaphragm, fuel on the other.  It's a flexible fabric that gets old and cracks.  A small crack can let a lot of unmetered fuel into the intake manifold.  Not uncommon.

I pulled the vacuum hose off while it was running and the idle cleared right up, I didn’t find any liquid fuel in the vacuum hose but it does smell strongly of gas

[Zed Head]

Try turning the engine off then pulling the hose.  Just to see if it's wetter, the engine vacuum won't pull through the hose with the engine off so any leakage should still be there.  The fact that it runs better when you turn the pump off is a sign that it wants a leaner mixture though.  Seems like you need a new FPR.

Edit - if the FPR is leaking you might see some drops at the hose nipple for the vacuum source, on the FPR itself.   Run the fuel pump with the vacuum hose off.

Edited June 18, 2019 by Zed Head