300ZX Fuel injection Diagnosis- by Steve Chong

Unlikely Causes

Could the problem be a stretched timing belt, causing the ignition to be retarded, hence hesitation?

I compared an new belt with one that had done 60K miles (my 91TT) and one that had done 80K miles (my 90 NA). I couldn't measure any difference in the lengths of the 3 belts. All three belts were made by the same company, Powergrip of Japan. The belt that had done 80k was so worn that every single tooth on the belt had a crack between the tooth and the belt backing. The back of the belt was also starting to show crack marks. The tensioner gap on my 91TT was still around 4mm, indicating no stretch. My conclusion is therefore, that a correctly installed cambelt will not alter the ignition timing significantly with wear.

Could the problem be the vaccum feed to the AIV (Air Induction Valve)?

The AIV is what feeds air into the main catalytic convertors. I deliberately pulled of the feed hose, located on the balancer tube on the right hand side of the engine, to the AIV solenoid. This allows an air leak through a 3mm (1/8 in) diameter tube. The most noticible effect was that the idle went up to 1100 RPM. I went for a test drive,but couldn't detect any hesitation. If there were other leaks or problems with the car, maybe I would have got some hesitation.
 

What my problem was

My hesitation problem turned out to be the timing belt incorrectly installed. Here's how it happened. When the car arrived from Japan, it was delivered to the port of Auckland. I was living 400 miles away at the time, so asked my father to pick it up, and he observed that it was leaking oil, a possible cause being the camshaft oil seal or front seal failure. A local garage in Auckland was contracted to fix the problem. Then when I got the car, I noticed the hesitation problem, and not being an intermittent problem, I thought it would be easy to trace. Because the problem always occurred in the transition from idle to higher RPM, I thought it must be air flow or fuel/air related- so started examining all the devices that control this.

After about 40 hours of examination, I decided to address the oil leak which had not been fixed. When I removed all three timing covers I found the timing belt on the cambelt drive sprocket to be one cog out, retarding the valve and ignition timing by 14.4 degrees.

A letter of complaint was drawn up smartly, and taken around to the garage, where there was a calm 5 minute discussion. Immediately and without any hesitation whatsoever, a full cash refund was given.

Incidentally the oil leak wasn't the seals at all- it was coming from two leaking inlet camshaft sprockets.

BTW, when I got everything back together, I deliberately retarded the ignition timing by 14 degrees to observe what would happen. I found there was a loss of power, and a very slight hesitation, but nowhere as deep or prolonged as when the valve timing is out also.