Engine running/power issues HELP, Need help diagnosing vacuum/electrical issue? Options

[beng295]

Hi all,

I have scoured the forums and any/all articles I could find out there without much luck. Any help of a point in the right direction would help greatly...

Anyway, I recently got ahold of an all original '72 1.7L (D-Jet, oil bath air filter, and all, with 61k original miles), and am attempting to diagnose a poor idle and LOW power.

Starting up cold idle is around 1.5-2k rpm for around 20seconds and slowly decreases down to a repetitive fluctuation between 400-1000 rpm (sporadic puffing from the tail pipe like a cylinder or two is misfiring). I can pull the electrical plug from cylinder 4 injector and no change to the way the engine runs so I am assuming this is at least one of the misfiring cylinders. Ignoring the idle issue, getting it out and driving it... is difficult as it has almost no power (no acceleration and I can barely get it backed up my driveway to pull it back into the garage) like it's only running on 2 cylinders.

I do want to note that I noticed the adjustment on the computer (assuming this is the fuel adjustment at idle - lean/rich adjustment) is turned all the way up and it will not idle at all if I turn this down to the factory marked position (it was like this when i purchased the car).

All vacuum lines are 2 days new, not leaking and routed correctly, vacuum advance/retard working properly (as far as I can tell), new cap/rotor, wires, plugs (properly gapped), valve lash adjusted on all cylinders...

Now here is where I'm personally confused and lost... If I pull the vacuum line from the MPS, almost EVERY issue is corrected (Idle is almost perfect, and power/acceleration is restored when driving around). I did take a quick look inside of the MPS and there appears to be no damage, and also as far as I can tell (without a vacuum tester) it does hold vacuum. I can plug the vacuum line back in and the idle increases by about 500rpm for a couple seconds and then immediately goes right back to running like garbage.

Am I thinking too deep into this issue or do all signs point to a faulty MPS? Thanks for any help you all can give.

[BeatNavy]

A disconnected MPS is going to make the car super rich under normal circumstances. In your case it sounds like it will mask a super lean condition. Other evidence that you're probably way too lean:

1. Big idle hunt, and
2. idle adjustment knob maxed out to rich (PO's attempt to fix condition?)

So I would start with troubleshooting fuel delivery. Among other things to check would be fuel pressure at 29-30 psi, fuel filter not blocked or a line kinked somewhere, and, most importantly in my book, clogged or non-functioning injectors. I'd recommend getting the injectors checked and serviced by someone like Mr. Injector. It's cheap, quick, and can reduce the likelihood that you'll be chasing your tail.

Having said all that, it's still worth doing full check on other systems and sensors, especially MPS, CHT, timing, plugs, etc.

Oh, and (IMG:style_emoticons/default/welcome.png)

More bedtime reading: https://members.rennlist.com/pbanders/DJetParts.htm

[Highland]

Have you checked the resistance on the CHT cold and when it starts acting erratically? I have a variable resistor in line with my CHT and I can make it act like you’re describing with very small adjustments.

Are the rest of your cylinders firing?

[beng295]

The CHT checked out fine. The issue ended up being a fuel delivery issue.

The PO had done something very strange. They had replaced the fuel pump which fed fuel into an Edelbrock fuel pressure regulator with 1 return line back to the tank and the main feeding line out of the regulator then went into a plastic Y (looked like a vacuum hose connector) and fed both of the fuel lines leading through the engine shelf... which for one rendered the stock fuel pressure regular useless (yes it was still there) and basically created a dead end loop with air bubbles sticking in the high spots in the line creating a "spongy" erratic fuel pressure at the fuel rails.

I removed the Edelbrock fuel pressure regulator and re-created the continuous flow of fuel again and utilized the stock fuel pressure regulator. This gave me a completely steady fuel pressure reading at the fuel rails.

This is when i also noticed they never bothered to put in a fuel filter and had some strange combination of adapters going from the 3/8" fuel line front the tank to the 1/2" nipple on the fuel pump (this combination of adapters also were also not meant for gasoline as the inside was all rusted and flakey). Guess what i found at the screen/filters on the injectors? (IMG:style_emoticons/default/blink.gif) im surprised they worked at all...

Anyway. Thanks to your help i got the lines re-routed properly, put in a fuel filter, and used the injector tester i had laying around to blow carb cleaner through each injector in both directions to get them cleaned out.

All back together properly and after re-adjusting the air bypass screw to compensate for the extra fuel i was getting now it runs like new.