Hi All,

My ’97 Corolla has intermittent hesitation. Feels like someone has the car on a leash and is tugging on it sharply from behind. Happens mainly during freeway driving at 60ish or moderate engine load, but not during overrun. When the fault occurs it can be temporarily alleviated by giving a little extra gas. Does not seem to be temperature dependant.

The vehicle has been to two shops, one of which replaced the spark plugs, and the other replaced the distributor (twice), the ignition harness, distributor cap, and the spark plugs. I have replaced the fuel injectors and fuel and air filters. The air intake path is clean and I’ve checked the MAP and IAT sensors. The car idles fine.

Fuel pressure looks good at 39psi (idle), 48psi with the fuel regulator vacuum hose removed and plugged, and 70psi with the fuel return hose temporarily pinched off.

The pressure checks did reveal one anomaly: according to the Haynes manual, the pressure should be 38-44psi with ignition on/engine off. It’s actually zero. There is no fuel pressure until the ECU detects that the engine is cranking or running. Not sure whether the manual is wrong or the car but I’m betting it’s the manual.

I have a Scanguage II permanently mounted on the dash. No scan codes are set and none of the twelve monitored parameters fluctuate when the fault occurs.

The problem persists and is getting worse.

Can anyone suggest where the fault might lie? This one’s driving me nuts.

Thanks.

Hi,

The behaviour of your engine appears similar to what I experienced in one of my Toyotas.

No shop was successful in correcting the problem. However, I found that the culprit is traced to a defective distributor vacuum actuator.

This part should open when you reach 70 kms/hour. When I have replaced the part, the drag (as if pulling a load) and jerk at that speed disappeared. The car run smoothly without pressing too hard the accelerator to attain speed.

Thank you for responding. Having had shops burn through my checkbook without finding the problem, I understand your comment. In my case, the vacuum actuator isn't the cause because the 97 doesn't have one. The distributor timing is not adjustable. Spark timing is determined only by the ECU. I'm going to pull the ECU today to investigate a possibly-related fuel delivery problem.
Thanks again for your input.