My Volvo was running great, when I started it to go get take out, it barely would leave the parking spot, it starts fine and idles, but will NOT rev over about 1100RPM, then it stumbles and will only idle. The intake is spotless, Tried a different Idle control switch, The only code was the IAC but that was replaced, also replaced the Fuel pressure regulator, checked for vaccuum leaks, checked all hoses, checked and cleaned the knock sensor, jumped the radio suppression relay, Pulled out the air filter. when the AMM is disconnected the idle changes slightly but it still won't rev, even changed out the AMM with an old one (Condition unknown) just hoping for some change in acceleration---got none, changed the fuel relay but both pumps are running, I have max pressure on the injector rail, Here's the kicker...I squirted raw gas into the intake through a vacuum line and was able to get acceleration ??? I'm at a loss, not wanting to buy an ECU ! Any other ideas guys ? Thanks...

 

Welcome to Volvo Forums !

 

Welcome to the forum Vstressed

 

What controls my injector opening duration ? It has to be gas/petrol feed-right ? I also changed out the coil, I knew it was OK but I had one lying around !

 

the ECU grounds the 4 injectors to turn them on, the width of that grounding pulse determines how much fuel they squirt. all 4 injectors fire together, so each squirt is actually 1/4th of the full fuel charge for that cylinder. The ECU uses the engine RPM as determined from the crank position sensor to know how often to inject, and the air flow (from the mass airflow meter), and the engine coolant temperature as a secondary input to determine how much to inject.

 

Thanks for your reply: So since it starts great and idles, would you change the MAF or the ECM first ? It will not rev past 1100 with or without the MAF connected...but will rev up with some gas shot into the intake !

 

the ECU's on these cars are pretty darn reliable, while the MAF's are a frequent source of problems.

catch-22, there's a lot of dodgy aftermarket MAF's out there. ideally, you want a /real/ Bosch rebuilt one, but these are kinda expensive.

 

Thanks, I will probably go with the MAF first, it's also cheaper, I should know right away if it allows the engine to rev past 1100 without bucking ! Although unplugging my MAF and putting it in "limp" mode still makes it "buck" if I try going past 1100 Rs ?

 

what sort of shape is your ignition system in? reasonably fresh plugs with correct gaps, proper quality (eg, bougicord) spark plug wires, good bosch distributor cap and rotor ? I dunno, the fact that it revs up if you squirt fuel in certainly sounds like its not getting sufficient injection.

a fuel pressure test would be appropriate, too. ideally, hotwire the fuel pumps so they run without the engine running, and the pressure should be 42-44 psi. if the engine is running, the manifold vacuum modifies the fuel pressure, so at idle 35psi or so is pretty normal (unplugging the vacuum line to the fuel pressure regulator would bring it back up to 42-44 psi).

 

If it doesn't rev up properly with the MAF disconnected, I would really suspect issues with fuel delivery. As Pierce noted, it is rare that the ECU fails on these. Check the operation of your fuel pumps. Are they both working? Check the fuel pressure as Pierce suggested. Diagnose first. Purchase parts second.

 

If it revs up fine with fuel being fed directly into the intake, that should mean all the ignition components are all firing properly ?

 

Yes. Since it revs up when you feed it fuel directly, I suspect it may be a fuel deliver issue. I would start by making sure both fuel pumps are working. If both are running, check the fuel pressure at the rail.

One caveat on this. To feed fuel directly, you must have removed the air intake. There is the possibility that there is some restriction in the air intake system.

 

I pulled a small vacuum line off the intake, behind the throttle flap, and squirted fuel directly into the intake, it revved normally ! Pierce's first post seems the most logical...the"Bucking" at 1100 RPMs almost seems like an uneven disbursement of fuel from the injectors ? Replaced fuel pump relay, both pumps are running, the fuel filter was removed and is clear ! During a pressure test, the pressure regulator which was also replaced, blew right off the fuel rail and shot gas over the fender...

 

what sort of pressure test was that??

a fuel pressure test is done by connecting a fuel pressure gauge to the fuel rail, and energizing the fuel pumps. if everything is working, the pressure should be 42-44psi relative to manifold vacuum (eg, if the manifold vacuum is -9 psi, then the pressure would be 33-35 psi). I would rev the engine and see that the fuel pressure remains correct regardless of the throttle setting (noting, of course, that opening the throttle changes the manifold vacuum).

 

I am pulling ECUs looking for problems now, I see many schematics for 92 Volvo 940 Turbos but non of them show any voltages ? I see the battery on them with a "red wire" which I assume is 12 VDC but non show any other voltages ? While trying to check the throttle position sensor I got a reading of like 3VDC ? I don't know if this is correct or not...does anyone have a link to a schematic with voltages ? Thanks...

 

there is only a throttle position idle switch, not a throttle position sensor. the switch is closed at idle, and open if not at idle.

the only analog inputs are the o2 sensor, the coolant temp sensor, and the MAF/AMM. the crankshaft position sensor generates pulses when the engine is turning.

 

I opened up the throttle switch, inside it has a micro switch that clicks closed when throttle is Closed and a set of contacts that close when throttle is fully Opened ... I did the switch test (with 2 different switches)at the code block and all I got was flashing constantly, no code ever appeared ? I wish I had a voltage I could test, since the throttle switch reports to both the IGN ECU and the fuel ECU, I still believe the injectors are NOT getting the pulse/throttle UP signal(idle is fine)...I am going to try tracing wires (Orange and Red/White) from it to the ECUs. I'm not happy with the throttle switch signals !

 

the throttle idle switch goes to ground, and is connected via a orange wire to pin 2 of the ECU, and also to pin 7 of the ICU. inside the ICU/ECU should be a 'pullup' resistor, so if the switch is open, the input is some voltage above zero (don't know, don't care), and if the switch is closed, it will be 0 volts.

ecu pin 3 is the full throttle switch (red-white wire).

 

I really think you are barking up the wrong tree. The TPS for the most part switches the idle circuit on when the throttle plate is closed. Since the car idled properly, I really doubt that your problem is related to the TPS. Also, as stated earlier, the ECUs on these rarely fail.

As Pierce noted, what kind of fuel pressure test did you perform? Just because fuel came out doesn't mean that it is supplying the correct pressure to operate properly. You really need to get a pressure gauge and test it properly. If you don't have the equipment, borrow it (Autozone will lend tools at no cost) or take it to someone who does.

 

Thanks, act1292...Checked both ECUs visually, they looked fine, cleaned all connections with contact cleaner, I am going to get the exact fuel pressure to put that to bed, it seems more injector related, if I force the throttle past 1100RPMs it bucks and backfires ? My resistor block is putting out 12VDC, shouldn't the Voltage drop across the resistors ? Thinking about disconnecting an injector and applying 12VDC and a ground to it, just to make it apply more gas into at least on cylinder. Also, I can't get the throttle position switch to give any test codes. Position 2, Key on, throttle closed, then partially open, then fully open...press button twice for one second (only flashes) ? Page 2 >>>