This is a write up about the 'hesitation problem', which some people seem to experience.  We had the opportunity to look at a 99 Civic Si turbocharged B16A with Hondata s200 & RC 440 cc injectors.  The Civic had a stumble at cruise which sounded like it was similar to reports of the hesitation problem.

First we would like to point out that there are a multitude of problems which can cause the same symptoms associated with the hesitation problem. The two main causes of any hesitation are rich part throttle fuel tuning and too little ignition advance.

Adapter Harness

First it felt like throttle tip in was dumping fuel so we datalogged and then looked at the results.

One thing about the Civic is that with an aftermarket clutch and stock engine mounts there was significant chatter when the clutch was slipping.  From the above graphs it appears that this was causing noise on the TPS input, which resulted in the ECU from throttle tip in. Changing the OBDII-OBDI adapter harness to our OBDI/OBDII ECU fixed the problem as below.

Closed Loop Operation

Looking at the closed loop operation it appears that the car was tuned very rich at part throttle.

First we used a wide band lambda to verify that the stock o2 was working correctly.  This is fairly easy to do: let the car idle and check that the lambda cycles around 14.7:1.  The stock o2 sensor was fairly good.  Next we progressively leaned out the fuel tables (25% less fuel  in column 1 to 10% less fuel in column 6).

Now the closed loop operation is better, but not perfect.  For comparison a stock vehicle will typically run between -5% to +5% short term adjustment when in closed loop. 

Battery Voltage

The battery voltage was cycling from the ECU controlling the alternator.

Removing the ALTC wire from the harness stopped the ECU disabling the alternator.

Other Tuning Changes

• Increased part throttle timing by up to 10 degrees. (Yes, 10 degrees at part throttle only).
• Set throttle tip in adjustment to 0.4 (A/F was 11.5:1 - 12.5:1 upon rapid throttle tip in).
• Set ECT fuel enrichment to 0.6 and the post start fuel enrichment to 0.6 (A/F was now 12.5:1 when the car was started from cold).
• The TPS sensor was reading 0.51V with the throttle closed, so this was set to 0.45V

Target O2 Voltage

A new feature in RomEditor allows us to set the target o2 voltage. 

Normally this is 0.5V, setting the target voltage to the limit of the stock o2 sensor (about 0.8V) will cause the engine to run richer in closed loop (air/fuel ratio about 0.2 points richer). This is a good idea for a turbo. The customer reports the EGT dropped significantly when cruising after making this change and advancing the ignition timing.

Fuel Pressure

A common problem with fuel pressure regulators is that the diaphragm runs out of travel, limiting the adjustment of fuel pressure under vacuum. This is a problem because the car will then run rich at part throttle. To check you will need a fuel pressure gauge, preferably plumbed into the fuel rail. With the engine idling disconnect the vacuum line from the fuel pressure regulator (plug the line with you finger to stop the engine rpm from increasing). You should see a 9-10 lb reduction in fuel pressure, assuming your engine is running at 20" vacuum at idle.  Usually you will see only a 5 lb drop in fuel pressure (sometimes there is no drop). This is because the fuel pressure regulator cannot decrease the fuel pressure. Often increasing the fuel pressure (and reducing the injector duration) will fix the problem as it raises the idle fuel pressure to more than 10 lbs of the fuel pressure regulator minimum pressure.

Conclusion

The Civic ran much better, but the customer still reports a slight hesitation. There are several areas with the Civic which need further investigation:

• The ignition timing needs to be checked on a dyno at part throttle as it seems that the actual advance is significantly less than the advance in the ignition tables. It is also worth substituting distributors to eliminate an ignitor problem.
• The battery voltage would randomly drop to 12V. Sometimes this was triggered by the brake lights, other times it was random.  Our concern is that there is a wiring issue somewhere on the vehicle which needs to be fixed, which may cause a drop in voltage to the injectors or the ignitor.
• The injectors had resistors as some point and the injector wiring was altered from stock. It would be good to check that wiring.
• The exhaust manifold had a crack which could be introducing outside air into the exhaust system upstream of the o2 sensors.
• The fuel pressure regulator was not reducing fuel pressure under vacuum.

If we find out any more information from the Civic we will update this section.

Recommendations

• Check all sensor readings for abnormal values.
• Disconnect the ALTC wire if the battery voltage drops below 13V
• Make sure the part throttle operation of the vehicle is well tuned. The short term adjustment should be from -10% to +10%.  -5% to +5% is ideal.
• If you're not concerned about emissions, add a lot of timing at part throttle.
• Try changing the 'maximum closed loop load' higher if you get hesitation upon throttle tip in. If this helps then the real issue is the part throttle fuel maps.
• Pull apart the engine bay wiring connectors and clean the contacts.  If they are corroded then file them clean as well.
• Check that the main ground to the thermostat housing is good, and there is no voltage difference between the negative terminal of the battery and the ground point.
• Disconnect anything tapping into the o2 sensor wire, including any autometer type lambda displays.
• Check that the shields on the wiring harness are not grounded at both ends. (The distributor wires and o2 sensors are shielded).
• If there are any devices tapped into the ECU wiring loom, consider disconnecting them and returning the wiring harness to stock.
• Check that the fuel pressure regulator has sufficient adjustment left to regulate fuel pressure at high vacuum.