This is thread 3 as I seemed to have chased two avenues that didn't pan out and I didn't want to confuse people in this one.

stage 1-replaced all ignition stuff, plugs,wires, coil, rotor,cap... nothing changed, but car needed it anyway so it's all good

stage 2-I didn't have fuel pressure gauge and took a guess at in-tank fuel pump due to some of the symptons, it didn't help either

now I'm at stage 3

issue: Car has crazy surging idle, cannot physically rev past 3k or so, sometimes it runs fine but lately it's getting progressively worse, sometimes stepping on the brake pedal makes it start surging but today for example it surges on its own at idle or in neutral/park etc.. the car is completely undriveable, even backfires at times and has no power.

- checked fuel pressure before the injectors, about 40 psi at idle, jumps a little but nothing crazy, sometimes 42, maybe 35 if I rev a little (it's a lousy gauge too and had a small leak, so there's that...). I believe this is perfectly fine

- hooked up vacuum gauge to a small hose going into the intake, while the car is surging it's jumping from about 5 to 10 , when at times it runs better I've seen 15. Could it be a vacuum leak? I've looked at every hose I can think of and see... small , large, IAC hoses...

- when I unplug MAF it stops surging and idles very well.

- I have tried eliminating larger hoses and replaced few questionable ones, the air intake elbow seems fine, tapping the MAF does nothing, I have cleaned the MAF

- something odd, I get 0 PSI fuel pressure AFTER the fuel regulator, any idea why? first I had leaks again but after fixing those the gauge just won't move, car is running/idling/surging even if I rev it I don't see the gauge move.

any friggin ideas??? I'm ready to torch the car Thank you in advance.

 

if it idles correctly with the MAF unplugged, thats a very strong indication that the MAF is bad. the MAF is interchangeable with all LH2.4 cars, which are pretty much 240/740/940 1989+ non-turbo and 1990+ turbo except the rare Regina cars and even rarer LH3.1 cars.

there's no fuel pressure to speak of after the fuel regulator, thats an unpressurized return line to the gas tank.

the fuel pressure at the fuel rail before the regulator on a LH2.4 car should be 42-44PSI relative to manifold vacuum. vacuum can be as high as -9 or -10PSI at idle, so the absolute pressure can be as low as 32-34 PSI. you can disconnect the vacuum hose from the back of the fuel pressure regulator to see what the pressure is without vacuum, or just jumper-wire the fuel pumps with the engine shut off to get this basic reading.

 

thanks pierce, guess I'm heading to a junkyard tomorrow and buying two MAFs just to not get another bad one.

also thanks for explaining the return line, I figured I'd see some pressure there, but that makes sense. I did the sniff test often recommended here on the FPRs vacuum line and while it has a slight gas smell it's not wet at all and it's not very strong smell (plus it's likely original vacuum line). Should I grab a used FPR too or is that pretty much eliminated?

You don't think it could be O2 related by any chance? I'm not entirely sure how to test it on these cars and what the voltages should be etc..

 

the O2 sensor is just a fine tune on the mixture for smog reasons.

it has 2 connectors, a 2-pin power connector for the heater, and a 1 pin signal connector. open the 1 pin connector, wrap one end of a single thin bare strand of copper wire around it and snap it closed again trapping this wire (I strip a piece of lamp wire and use a strand from it, maybe 2" long). take a volt meter, put it in DC Volts, and set the scale/range to the lowest setting that will measure 1V... hook the red probe up to that thin copper wire, and the black probe to a good engine ground, like the fuel rail where the ground screws are (clean the screws). start the car, let it warm up... the voltage reading should swing lazily between around 0.3V and 0.7V and back, every 5-10 seconds. if it does that, you're in lambda balance. gently revving the engine to like 2000rpm or whatever, and the swings back and forth should be somewhat faster but the same voltage range. the exact voltage isn't important, the 'low' value can be 0.1-0.3V and the 'high' value can be 0.6-1V, whats important is that its cycling above and below 0.5V....

 

so it wasn't a MAF, got a used one from 89 940, looks slightly different as far as the internals where the wire goes, but same 6 wires, bosch etc.. identical issue after about 1 minute of idling.

What else is there to check??? could that ignition module cause this? (the one by the airbox)

I will check the O2 tomorrow, I read the ECU will start reading O2 after a minute or two of running, it really ran pretty good initially but once the heat starts kicking in that surging idle begins

this is starting to get depressing.