1990 Volvo 740 turbo.
My car hesitates on acceleration. Very bad on hills. Same whether engine is cold or warm. Kinda feels like firing on 3 cylinders...??
Idles ok and when I rev the engine in park ok.
Things I and shop have done:
Newer plugs, wires, new dist cap and rotor, new MAF (was throwing code for it and still comes up once in a while), injectors tested ok (they all click when in use), valves adjusted, replaced intake manifold gasket, replaced crankshaft position sensor, replaced fuel filter, cleaned throttle body and put on new gasket.
Haven't detected vacuum leak.
compression test: 165, 165, 170, 170
Anyone have a clue what could be causing this?

 

Hi and welcome top the site. I have moved this thread to the 740 section.

 

Is the problem worse when it is below 1/2 tank of gas, and less if the tank is full? if so, I'd be checking the in-tank fuel pump.

its also possible the main pump is weak. or the fuel pressure regulator is going... rather than just blindly replacing, I would TEST these things.

btw, just because your injectors click, doesn't mean they are 'good', it just means they aren't totally bad. to properly test injectors, you have to remove them from the car, hook them up to a test jig that pumps 42PSI of fluid pressure at them, ideally using something a lot less flammable than gasoline, and then fires the injectors so you can verify the spray patterns.

 

It could be a bad Cat. They will run okay without a load but once driven with a load, they will stumble and feel like half the powers missing.

You didn't mention replacing the air filter also but I'm sure that must have been done also.

 

silly question, but have you verified that the throttle linkage, cable and pedal are all working properly? have someone press the pedal to the floor (with the car off and stopped, hood up), and watch the throttle linkage to make sure it travels the whole way and back smoothly ?

 

What kind of MAFS do you have in there? You say you replaced it but many rebuilt units are bad and give substandard performance...

Also, I second the "bad cat" theory, similar to the "banana in the tail pipe"; if your exhaust line is not flowing, you'll get those symptoms...

 

I have replaced the air filter, had the injectors tested (they were ok). I bought an aftermarket MAF. What can i say....it was cheap $50 compared to 450 that the shop wanted. When I disconnect the MAF the check engine light comes up right away which it doesn't when connected.
By the way the car wasn't throwing a code for the MAF until I took it to the shop.
Originally had code for 2-1-4 and I replaced the crank sensor.
Doesn't matter how much gas in tank.
I'm thinking checking fuel pressure next. CAT after that?
By the way I have had this in a shop 2x and they haven't been able to figure it out.
Thanks for the suggestions.....

 

find another LH2.4 240/740/940 (that would be anything 1990+, or a 1989 non-turbo, they all use the same MAF) thats running fine, and temporarily swap their MAF with yours (2 minutes with a screwdriver). if the problem goes away, your $50 MAF is junk. the cheap ones that work are $200, not $50.

 

The problem is finding one to swap with.

Forgot to mention that all plugs looked normal when pulled.

 

New info: resistance checked ok on coil.
Can't check fuel pressure due to no schrader valve on fuel rail and can't find any adapters that will screw between fuel line and rail.

 

when it misses, does the tachometer twitch? if so, then its an ignition or CPS thing.

OH. my 740T, the big silicone rubber air hose from the turbo air inlet, under high throttle/boost, it was getting sucked into the mouth of the turbo, and letting in too much air in, causing an immediate loss of power. attempts at re-clamping the hose to the turbo air inlet weren't very successful, the hose end was damaged, so I got a new hose, problem solved.


1991 940SE turbo


I'm talking about the lower hose in that picture, that goes behind the wastegate actuator.

this caused the boost needle to twitch during the power loss. really, any air plumbing issues between the MAF and the intake manifold will cause serious power loss during boost.

 

I'd say first check vacuum leaks, then check exhaust leaks starting from the catalytic converter and moving up towards the engine. These two problems alone can cause a lot of issues with driveability.