tosha

Hi folks, long ago I posted about hesitation on sudden acceleration at reasonable RPMs (above 3500). I've changed my acceleration style to be a bit smoother initially (also to reduce LWF transmission-vibration noise), and made many fixes that make the car more responsive, and almost never experience the hesitation anymore. I fixed crossed distributor wires, installed LWF/RS clutch, RS engine mounts, new fuel pump, new gas tank breather canister, PSS10 with settings I love and my cab feels like an eager sports car now.

I avoid the "hesitation" now with smoother initial throttle, but it does happen sometimes. When it does, I often hear a ping/knock that I was too ignorant to catch in the past. I am definitely not lugging the engine in second gear at 3500RPM.

I wonder if the root cause of the knock is the root cause of the hesitation. I'm writing today because this afternoon I got a knock/ping when I quickly blipped at speed to go from third to second. I wonder if the knock sensor is triggering a fuel/air mixture change that briefly results in the weird hesitation I've been feeling on harsh acceleration. But why would I get a knock under normal blipping, or harsh acceleration at a reasonable RPM?

Any hypotheses on why I might get a ping/knock when blipping to downshift?

TJ993

It would be helpful to know if Varioram car or not.
My 97 has had a similar issue fuel pressure regulator after replacing fuel pump.

Previously Vacuum issue > CEL On > SCAN showed CODES 80 / 208 USING “PIWIS” TESTER

PERFORMED SMOKE TEST / SHORT TEST / AIR LEEK TESTS & Found a DEFECTIVERESONANCE FLAP – PART # 993-110-462-02 –
V RAM NOT GETTING ENOUGH AIR = CEL (GENERIC P0410)

CEL - DME RETARDS TIMING = PINGING and ENGINE SLOWLY CUTS BACK ON POWER DELIVERY.
Replaced Resonance Flap and wow - car has power.
Hope that helps? If No V-ram carry on
TJ
TJ

Steve Weiner-Rennsport Systems

Check to see if the distributor belt is intact or not.

pp000830

The sounds, I would think, are unspent fuel making its way to the exhaust system when the ignition doesn't take place momentarily during the hesitation you describe. In my mind, this would lead me to believe the issue is not with fuel supply but with the ignition.
Beyond all the obvious stuff:

• caps/rotors

you could check the less common items:

• The distributor belt,
• both ignition coils,
• Idle valve - cleaning
• If your wires are old, the wires,
• I would even check for oil overfill potentially fouling the plugs
• Oxygen sensors slightly out of specification, something that requires a detailed analysis of the signal they are providing, not something easily done by an enthusiast.

Unfortunately, the root cause may simply be the lightweight flywheel who's influence on the issue probably varies by slight changes in running conditions.
Andy

rk-d

^^^ I wouldn’t think the LWF would cause the problem but perhaps rather exacerbate the conditions that could contribute to the problem. When I had my LWF installed I was sure to refresh the ignition system(plugs/wires/distributor rebuild) and vacuum lines and ISV/SAI check valve so I wouldn’t run into stall/erratic running issues. Probably overkill, but maybe not.

tosha

It's a '96 with varioram. Is there a simple way to rule out the fuel pressure regulator? When I first reported this issue to a shop I no longer use, they drove around with a pressure gauge, but I'm not convinced they understood how to reproduce it. I could try this again with a friend.


This is exactly how my problem feels, but I have no CEL.

This is interesting. The ping is quite distinct, just like the sound before a stall. Does that rule out the idea that it's unspent fuel in the exhaust system?

The distributor was changed to a Bosch-rebuilt one and installed by the guys who crossed the wires. Wires and coils were changed at that time, about 15K ago. I'm not sure the hesitation existed before the new distributor was installed. I had just gotten the car, the front left shock was leaking and handling was horrible, so that racing shop did the 90K maintenance. Could the problem lie in a poorly-rebuild distributor?

Definitely had overfill a few times, but not for long. The plugs seemed clean when I took the engine down for LWF ~8K ago.

This is interesting. I have access to a hammer and can observe O2 readings. What should I look for?

The problem existed before the LWF and was actually more pronounced. I wonder if the shorter engine latency actually narrows the time window in which this can happen.

pp000830

See my notes below:

tosha

Does anyone know how to interpret these numbers from a hammer? I would have expected USHK and USHK2 to both hover around 66 as the after-cat sensors. Could that cat cell be dirty? I had crossed distributor wires for a bit (my mechanic told me I was lucky it was post-detonation, but there was probably unburned fuel)

Steve Weiner-Rennsport Systems

Using that Hammer, see what the knock counts are when you are driving the car.

TJ993

Follow Steve Weiners recommendation.
Put this aside, for now > Hose from Fuel Pressure Regulator pic
Andy's suggestion > check Dual Distributor belt especially after replacement to ensure in proper working order is good peace of mind.
You really want to rule any distributer issue out
TJ

Ed Hughes

I think the belt suggestion was first made by Steve.......

The ping comment took me to one side of the ignition not firing.

abiazis

Great list to check......

tosha

Hi everyone, I installed the before-cat O2 sensors and immediately my idle RPMs dropped and the car drove much more smoothly. In fact, I scared myself a few times with the acceleration. I haven't yet been able to reproduce a knock, but I will try in the coming weeks and will drive around with a hammer and report back. In the mean time, the difference is perceptible.