the resistors limit the current when the injectors are on. when the injectors are off, you'll see 12V since there's no path to ground. if you could somehow power the injectors always-on, then you'd see a lower voltage (equivalent to the ratio of the load resistor with the injectors resistance... its a simple R divider ladder circuit. actually, I think it would be 12V * Ri / (Rl + Ri) where Ri is the injector resistsance and Rl is the load resistor... or something like that, I'm tired and I'd have to draw it to calculate it correctly).

I wonder if you don't just have some bad injectors that aren't spraying fuel properly. I had to replace the injectors on a mercedes 300E, that car had sat for years undriven, and most of the 6 injectors had bad spray patterns. new injectors with new seals made it run like a new car again. they CAN be tested, but you need a test jig that pumps a solvent through under full pressure, and energizes them with each injector into a seperate test tube, so you can see the patterns and how much fuel they are spraying. diesel injector service places can do this but might not be setup to deal with gasoline injectors, I dunno.

 

Thanks, but this is a car that ran great 'till I started it up the same night to go for take out ! It started right up (continues to start right up), then as I pulled out of the parking spot, it just stumbled and hasn't gone past 1100RPM since ?

 

an engine needs compression, the right amount of fuel to air, and spark at the right time.

a compression test will tell you if the mechanical aspects of the engine are in OK shape.

a fuel pressure test will tell you if the fuel delivery system is adequate. a timing light will tell you if the spark is going off at the right time, and the condition of the spark plugs will also tell you a fair bit about whats going on.

oh, hey. my 92 740T had a problem once where heavy footed acceleration would suddenly loose power, it turned out that the silicone rubber duct into the turbocharger air intake was rotted from past exposure to oil leaks, so when the turbo was gulping a lot of air, it would suck the back/lower side of the hose in, making a big air gap where air would bypass the MAF, and blam, power would die. back off and it was OK. I had to replace that big rubber hose.

this is the lower of the two big air hoses here,



and where it was failing was on the bottom side of where its clamped to the turbocharger air intake. the top looked fine. when I removed the old hose, it was very evident that the rubber was all rotted and mushy.

 

Hoses are OK...took a break to finish my taxes ... :P

 

OK, I had to have a cracked tooth extracted, so back to the Volvo...I did the fuel pressure test and it has 40+lbs. so it looks like the injectors are just not allowing gas through when it revs, it still starts and idles, (Remember I can inject gas directly into the intake through a vacuum line and get it to rev!) I pulled both ECUs and looked for any corrosion or contact problems they look like new, cleaned all pins and contacts while I had them out ! So I might just take a stab at a MAF sensor...? Mystified !

 

OK, today I replaced the Mass Air Flow with no change in symptoms....??? ECU next ? This is nuts...

 

the ECU and ICU are extremely reliable in these cars, any failure leads to a totally dead car.

 

Inspect rpm sensor. Common failure

 

Take a very good look at your turbo intake hose with it OFF.

 

Don't burn car to ground!

Disable ignition, pull fuel rail, reground wires and crank engine and watch injectors spray. Its a free dirty way to check fuel system. I only started one fire this way! spark plugs were firing. Ha. Disable ignition.

Or.....find green book and diagnose it the right way. Search. If you are lucky I will give you a link soon.

 

I already changed RPM sensor . The harness looked frayed...

 

Will try this, I have a fire extinguisher and a garden hose standing by ! Remember I was able to throttle up by squirting gas into the intake through a vacuum line behind the throttle plate...Also sprayed some gas during pressure test...Oops, I did start a fire at the distributor spraying starting fluid, while looking for vacuum leaks !

 

Thanks !

 

Backfiring means you are running very lean, or have ignition or timing belt issues.

Not revving above 1k.... you should triple check your timing belt and distributor timing. EDIT-since you have a head mounted dist (940) there is no way your dist is out of time. You should, however, double check timing belt marks.

 

I posted that it starts fine and idles, also when I squirt gas behind the intake flap through a vacuum hose I can rev it up ... that tells me that I don't have a timing or spark issue !

 

no it doesnt. wierd stuff can happen when belt timing is off.

Your timing is "probably" ok, but have you bothered doing a compression test ,like pierce mentioned, yet?


Have you pulled your MAF-to-turbo hose and inspected it?

 

Sorry guys, I have been away, I went to the junkyard and bought both cpu's... now this is weird...the replacement cpu's allow the engine to start and idle smoothly, I can now rev it as high as I want in park or neutral (would not rev past 1500 before) Here's the kicker: I put it in gear to go it WILL NOT accelerate !!!

 

go back to the beginning of this thread, and review everything again.

do a fuel pressure test.

verify the camshaft belt timing is correct.

do a compression test on all 4 cylinders.

verify there are no air leaks in the intake path from the MAF to the turbo to the intercooler to the throttle body to the intake manifold to the head.

 

Is there a signal to the CPU from the speedometer, the brake switch, or something on the transmission ? Why would the engine idle and rev up to 6,000 RPM perfectly and stall out as soon as it is put in gear ? PS: I drove it in stop and go conditions for half a mile after replacing the CPUs without any problems and then THIS ?

 

the main inputs to the ECU are the timing pulses from the Crankshaft Position Sensor (CPS), the engine coolant temp (ECT), the mass air flow (MAF/AMM), and the idle switch (so it knows to maintain idle)

last (non-volvo) engine I saw that could rev up in neutral, but totally bogged gutless under load had no vacuum advance. the LH2.4 system does its ignition advance electronically, based on the speed of the crank position sensor, and a load value coming from the ECU, so obviously /thats/ not it.