The car started dieseling after it is warm. Not for very long, just a second or two, but it is annoying. Anyone know a common cause for this?

L28, 4 screw SU's, petronix ignition.

Probably running hot, lots of carbon deposits in cylinder or your plugs are too hot. Couple of questions; what is the part number/heat range of your plugs, what temp thermostat are you running and has the cooling system been maintained and the coolant flushed recently?

Mine does this when I feed it cheap gas. It doesn't do it when I use Sunoco or Exxon, but when I fill up at the local 7-11 I get the run on.

Get the hihest octane pump gass you can find. Also check that your timing is not advanced, and also that your idle is below 900. My higher compression engine will diesel if either or both of these conditions apply.

I remember my 260z with flat tops was bad for that. Go to a colder plug, and check the idle. The 72 round tops never had this problem. I also had the vapor lock problem at the same time with the flat tops.

Ok, you guys hit a few of them that I will try.

I run the cheapest gas and lowest octane I can find. I heard some pinging on some spirited driving the other day, so my next tank was going to be a higher grade. The timing is spot on, but I am idling around 1100. I will crank that down a bit too. Thanks for the advice, I'll update once I run through the tank and make the tweaks.

Edited September 18, 201113 yr by Healey Z
im an idiot

I used to run NGK BP6ES plugs with no dieseling. Then I started running BPR5ES plugs which are Resistive plugs to eliminate the popping noise in my new stereo I bought. Since I installed the 5ES plugs, It's diesels after some spirited driving.

So I'm guessing the BPR5ES is a hotter plug? It's the only thing I've changed before the dieseling started.

Also, with the new cam in, I'm idling around 750 to 800

Dave

Yes, the lower number means hotter plug in NGK-talk.

http://www.ngksparkplugs.com/techinfo/spark_plugs/partnumberkey.pdf