Yet another EFI help thread :/

[dakinez]

I think I've tried everything I have read about. Here is the problem:

Car runs like silk when cold, like crap when hot. It backfires through the intake, stumbles a lot, will only build rpm with a lot of feathering, but when it's high in the rpms there is power there (I can burnout).

I'm looking for suggestions on what to try next. Here is what I have done/tried:

• New plugs
• New plug wires
• New dizzy cap & rotor
• Set timing to 14 deg
• New Fuel pump & Fram filter (between tank & pump)
• New main filter
• Tested fuel pressure, it's a little low at approx 33psi at idle, but goes as low as 24psi and as high as 36 as rpm fluctuates
• Cleared fuel lines
• Replaced throttle position sensor & connector (confirmed good)
• Replaced water temp sensor plug
• Replaced thermotime switch plug
• Replaced cold start connector
• Replaced Air regulator connector
• Cleaned all injector connectors
• I have seafoam in the fuel (interested to hear if this could be an issue?)
• I did a seafoam smoke bomb clean

I have also gone through the EFI bible and done all the circuit tests, everything is within range.

The fact it only seems to be a problem when hot suggests to be its a fuel related issue, rather than ignition or vacuum leaks etc (I can't find any vacuum leaks)

I'm all about testing things rather than randomly replacing things - as a first choice anyway.

Any help much appreciated!

 

Edited March 11, 2018 by dakinez

[Zed Head]

That is a very common problem with the 280Z's.  For some reason they tend to run lean in the mid-range.  Maybe they were tuned to the edge for emissions, and the fuel has changed, or maybe the AFMs or ECUs circuitry has drifted over the years.  But many do it.  It seems more AFM related, in my experience.  Different AFMs behave differently.

But, the cure is simple: add resistance to the coolant temperature sensor circuit.  Blue aka 240260280 supplied a writeup on the atlanticz.ca web site years ago.  Many of us put the rheostat in the cabin so that we can tune on the fly.  You can run wire through the firewall to the bullet connectors.  The coolant temperature sensor is the one with the two prong injector style connection, not the single prong.

http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

Your fuel pressure is correct, it's actually a little high at idle.  28-30 is more normal.  But 36 when there's no intake vacuum is right.

 

[dakinez]

Of all the hours I've spent looking at AtlanticZ articles I've never seen that one, I can't understand why!

Thanks for this, I'll give it a go over the next couple of days and see how it looks!

[dakinez]

I got this done today. I used a basic 1k potentiometer and spliced it in per the AtlanticZ article. Things are definitely better, I run it at full resistance, and the backfiring through the intake has mostly gone away. I was expecting to fine tune it, but it basically just needs to be at max.......

The problem I'm having now though is smoke. And its pretty dark, so I'm fairly sure it's running very rich now.

Is it likely that I have simply masked an underlying issue, or should I persevere with fine tuning the potentiometer? Any other tests I can do on components & circuits beyond what is in the EFI bible?

Thanks!

[Zed Head]

Seems like your backfiring might be from other causes.  But, when these engines run rich your eyes and nose will let you know for sure.  Oil smoke smells like oil, running rich smoke smells like gasoline.

[Reptoid Overlords]

Does increasing the resistance on those potentiometers lean the mixture or enrich it? It's my understanding that the front firing is caused by a lean condition.  Have you road tested it in this state? I like to get a friend to follow me every now and again as I run through the RPM's on the road to check for any smoke I might not see from the mirrors.  I dread the day it starts smoking going down the road.  I think I would dial the resistance back and see where it quits smoking.  The original problems might come back, but at least you'd be on the right track.  It was stated before that your fuel pressure was a bit on the high side.  Perhaps verify that the fuel pressure regulator is working properly.  Also, I see where you cleaned a bunch of connectors, which is always good, but did you clean the connection from the harness to the ECM under the driver's side kick plate?  It has been suggested by many members more experienced than I to use Deoxit D5 to clean all of those contacts/connections.  It can be found at Guitar Center music stores(It's pretty fun shutting a salesperson down by telling them it's for your Z car) It does work quite well as I've started using it on all sorts of electronics.  Don't forget to clean/inspect the grounds from the harness to the intake manifold.

[Captain Obvious]

Increasing the resistance increases the fuel.

And there's absolutely no way you should need an additional 1K worth of resistance to get it to run right. There is something else way wrong somewhere else that you're trying to compensate for with that resistor. No way, no how.

[Mike]

What kind of gas are you using? 

[siteunseen]

This happened to my '77 so I always mention it but so far it hasn't been a problem for anybody else but...

Where the EGR tube connects, under the intake manifold closest to the firewall on the driver's side, there's a clip that came out and that same tube comes off the hot exhaust.  After some years mine rotted the aluminum out and caused a huge vacuum leak that wasn't visible until I did the "yogurt cup test" for vacuum leaks and then used a mirror to look under there.  I found it and had to replace my intake.  Here's some pictures that kind of show what I'm talking about.  The mirror picture was to show someone the bolts for the heat shield but you get the idea. 

Hopefully you won't have the same problem but it's worth a look.  

[dakinez]

Gas is somewhat fresh 89 octane. I ran a lot of seafoam through, when that was done its on to just clean gas, didn't seem to make any difference.

I'm really struggling with what to look for next. I'm going to post up a vacuum vid, 

 

 

[siteunseen]

Sounds like the valves are too loose or the timing is low, maybe both?

vacuum looks okay though.

[dakinez]

52 minutes ago, siteunseen said:

This happened to my '77 so I always mention it but so far it hasn't been a problem for anybody else but...

Where the EGR tube connects, under the intake manifold closest to the firewall on the driver's side, there's a clip that came out and that same tube comes off the hot exhaust.  After some years mine rotted the aluminum out and caused a huge vacuum leak that wasn't visible until I did the "yogurt cup test" for vacuum leaks and then used a mirror to look under there.  I found it and had to replace my intake.  Here's some pictures that kind of show what I'm talking about.  The mirror picture was to show someone the bolts for the heat shield but you get the idea. 

Hopefully you won't have the same problem but it's worth a look.  

 

Thanks for this, I'll check it. I did go through all the EGR equipment as much of it had been removed by the PO and the car barely ran at all back then. But I don't remember checking that, so I'll make sure.

I did notice I have tiny exhaust leak from cyl #6 where the manifold bolts up to the head, the last bolt is missing; so i need to fix that but I don't feel like it's the cause of this current condition.