308 QV hesitating under accelaration

IMO it would be a very unusual fuel filter problem if it works OK at 3~5K RPM, but has a problem at just 1500-2000 RPM.

Are you saying that the problem is only present if the engine is also under load (i.e., if stationary, and just gently increasing the engine RPM, does the problem occur)?

And please confirm your version -- is it a standard (aka euro) 308QV? (K-Jet without Lambda?)

 

OK, based on your "only under load" report (i.e., only during a high intake manifold absolute pressure condition), I would first make sure that the small intake vacuum lines going from the intake system to the Warm-up Regulator and to the Ignition ECU(s) are in good shape, properly connected, and are actually presenting a vacuum at no-load idle.

(Assuming that it's not something as simple as a bad vacuum line/connection -- and it hardly ever is ) Both of these gizmos are supposed to change their behavior based on the intake manifold pressure:

The Warm-Up Regulator should add enrichment (i.e., change the control pressure going to Fuel Distributor) when the intake manifold pressure increases -- so this is something that really would require using the K-Jet pressure gauge set-up to confirm/deny (and IMO this is the most likely suspect based on the information so far).

The Ignition ECUs use different timing vs RPM maps for different intake pressures (but I would think that a "wrong" map would be more "sluggish" or "pingy" rather than cause an RPM dependent "stumble").

Just some suggestions -- let us know how it works out...

 

I have no idea what it is, but I do have a few questions to help others diagnose it:

Does it happen in all gears (well, 1st and 2nd at least)?

Does it matter if the car is hot or cold?

Does it happen every time or just sometimes under load?

Is it RPM related; i.e. if you are going 60 MPH in a higher gear and then downshift and gun it to redline, will it do the same thing or is it only at low RPMs?

It sounds electrical to me; a fault with your ignition perhaps?

Ken

 

I would suggest checking the hose that goes from the throttle body to the lower part of the WUR (control pressure reguator). This line give the regulator a manifold vacuum reading, so it can adjust fuel accordingly. Look for splits, or cracks, as they will give the regulator a false reading, thus causing the wrong fuel input. The upper hose is for altitude monitoring, and wont make a lot of differance. All hoses, gaskets, etc must be sealed in order for the Bosch system to work properly, as a leak can give the distributor a false reading on how much fuel to give. Air sneaking in leaks will not move the flapper, which meters the fuel according to air flow. HTH.

 

are their any ways to test the vacuum related stuff at idle? For example, sticking a vacuum gauge on one of the plenum ports or using a hand vacuum pump to change the vacuum to the WUR or the ECU.

What's a typical vacuum value at idle?

How about a smoke test...I assume you shove the smoke into the throttle body with the throttle plate held open and see if/where it comes out. I don't know if a smoke tester can be gotten cheaply enough to make it a viable tool for home use.

 

Here's an old standby check for vacuum leaks, it's based on the fact that vacuum leaks lean out the mixture:

Put an extension tube on the head of a can of starting fluid. With the engine ldling, spray a puff of ether on the vacuum ends, & anywhere else you suspect the leak might be.

When you hit the leak, the engine will run smoothly long enough to notice as the starting fluid will richen the mixture.

Be careful to not spray a big fog as it can confuse your test results, and more importantly, it's very flammable.

 

hi verell,

in your experience, is the lamda circuit fast enough to compensate for adding start spray or other combustable gas into the intake so you wouldn't notice it that much.

In other words, do you recommend disconnecting the O2 sensor when searching for vacuum leaks that way?

 

Umm, good question.

I've successfully used the test on other cars with L-Jet & Motronic EGI & it worked fine, those systems run closed loop lambda at idle.)

My K-jet experience is with Euros that don't have lambda.
My guess is yes as the K-jet has enough mechanical elements in the control loop that I'd expect it's response to be slower than the all electronic systems.

Of course you have to be having some rough/uneven idling due to the leak to start with. The uneven idling would mean that the leak is either outside the ability of the lambda circuit to correct, or else is causing the circuit to hunt for a stable point.

 

My ’83 Owner’s Manual recommends replacing the plugs every 30,000 miles (48,000 km) under normal conditions, or every 15,000 miles (24,000 km) for severe duty. My trusted mechanic told me every 30,000 miles should be fine.