Every couple of months a Z car owner somewhere cries out "Help, my car is suffering from hesitation/stumble: what could be wrong?" . These types of problems can be a pain to track down, and if the problem is an intermittent one, it can be months before the cause is found. With my 1991 TT, In 1997 I was on a 400 mile trip with about 50 miles to run. I decided to pull over in a small town for a break. As I was part way through a U turn, doing about 5 mph, the engine stumbled and cut out. Power steering dropped out, but luckily I was able to bring the car to a stop just in time to avoid smashing into the footpath. Started the car again, and you know, this problem didn't happen again for 3 years ,when I tracked it down to a faulty fuel pump relay.

Then I had another problem with a 1990 NA, which had mysteriously developed a deep hesitation whenever the car moved off from rest. The engine would idle perfectly, but as soon as you put your foot on the gas pedal, the revs would drop from 800 to about 500 rpm, then slowly build back up then from 1000 rpm everything would be fine. Whenever you took off from rest, the harder you jumped on the gas pedal the deeper and longer the hesitation. At higher revs the engine seemed to be a slightly down on power. Thinking the problem was related to vacuum, fuel delivery and improper fuel air ratio, I spent a lot of time looking at things in this area, but as it turned out, the problem wasn't in this area..

The idea of this article is to shed some light on what might cause electronic fuel injection malfunction,  with some thoughts on what to do next. You may decide to fix it yourself, or you might want to profile the problem to enable an expert to pinpoint the solution quickly.

If you are going to fix it yourself, then you really need to invest in a service manual because it has the detailed diagnostic tests you can do, and a more detailed description of components which can go faulty- about 190 pages worth.

While this article concentrates on the 1989+ 300ZX, principles outlined here also can apply to earlier Z models, and to other electronically managed cars.

Page numbers referred to in the Nissan 300ZX Service Manual are taken from the 1990 edition in English. Page numbers may vary according to the edition.