I have a 91 camaro with a 350. Since I cleaned the k&n airfilters and change the brake pads and fluid the engine runs okay when cold but runs like a dog when hot. It misses and stumbles. If I switch off the engine when hot it is very hard to start, like the timing is well out and kick backs the engine.

The service light is not on. I checked if any codes were on the computer by jumpering A & B. and it kept flashing - flash pause flash flash which I take to mean code 12? Is that how the code system works?

Anyone got any ideas what to check?

 

The problem you mention "lack of power" when the engine is hot is caused by a combination things on an SD L98. There are some "mechanical fixes" but to truly make the engine run consistently, you need to get into the eprom.

Mechanical fixes are simple, TB Bypass so the coolant doesn't flow through the throttle body will help. But if you go inside the eprom and recalibrate the MAT to work properly when it is relocated will help immensely. Also, adjusting your fans to come on much closer to the rated temp of the T-stat will keep the engine temperature more consistent (and power).

I have tried all three commonly used T-stats (stock, 180 and 160). I found with proper tweaking, a 180 works very well. I turn on my Fan#1 at between 185-190 (it's in Celsius so I don't have the exact *F conversion handy) and turn it off at around 184*. This means my engine seldom varies from 180-190*.

Lastly, heat will trigger extra "knock" and the knock sensor will pull a lot of timing out of your engine. I have done a lot of experimentation in this area and found minimizing the amount of retard the engine will pull out while in WOT to only 1* and working on my spark tables to give the most "effective" total spark advance (total spark less knock retard) really helps in keeping the engine's performance consistent.

It's involves burning your own eproms as it requires a lot of testing (no eprom writer is going to put that effort into your car unless you want to pay big bucks), but if you work at it and get inside the eprom, you CAN make your engine perform consistently from "cold start" to "hot restart". The only variation you will have will be due to ambient air temperature itself - which you can't really do much about. An ambient air temp of 40*F should produce about 6% more HP than an ambient air temp of 100*F. That is just due to the fact that 40*F air is denser than 100*F.

But on a basically stock L98, 6% only equates to about 15 HP, not a great difference. The problem you are mentioning makes the engine feel like you lost a cylinder (or two).