Intermittent Miss While Boosting
 

I'm somewhat new to the rx7 scene,but I've got a 1990 with an s5 t11 swap.The car runs great some of the time.But when its cold it has an extreme hesitation only under boost,its all stock,I've checked tps,engine and body grounds,visual for vac. lines,It seems like its starving for fuel,but if I run it for 10-15 min then let it sit for awhile(20 min),it runs really strong boosting perfectly.Any help is greatly appriciated.Thanx Chad

when you say extreme hesitation, what do you mean? like it stumbles at a certain rpm? You know that when it's cold the secondary throttle plate movement is significantly hindered, so much less air gets to the engine when you apply more than slight throttle.

You should always let the car warm up before applying right foot.

How much boost and do you have an FCD installed?

I'm only running stock boost,therefore I have no fcd.When you say let it warm up are we talking 20min of driving?Even when it comes up to operating temp it still has a huge 'hesitation',unless I park it for 15-30 min,then get back in.(it seems if the temp gauge is up at all when I fire it up it runs great,)I'm thinking faulty coolant temp or something like that.Not sure if anyone has experinced this or not.Thanx for the help.Chad

Does the hesitation seem to occur between 3500-4500 rpm?

I would say between 3000-5000,But it jerks and back fires so much I let off so I'm not sure if it will clean up and run right after 5000.Thanx Chad

If the exhaust & intake are opened up at all, you could be boost creeping into fuel cut at 8.6 psi. Do you have an aftermarket boost gauge, or are you using a stock one? Stock boost on an S5 is already 7.5 psi, so it doesn't take much.s

I've got a k&n filter with stock exhaust,(but I punched out the converters last year).I'm running a stock boost gauge for now but I have purchased a new one I just haven't installed it yet.So what is my next step?fcd?ported wastegate?Thanx Chad

Ok, you are hitting fuel cut. A hollowed out exhaust will still be able to overwhelm the wastgate. Make the next step porting it. There's a decent amount of metal that can come out of the S5 wastegate ports. I scaled mine up to 23mm & 26 mm (from 18mm/23mm), and have no boost creep with a fully open exhaust and intake.

An FCD without fuel mods will just let you overboost the engine and blow it, so don't worry about that part unless you want to start getting more power out of it. The FCDs also prevent the ECU from retarding timing under boost, which can lead to detonation as well. An rtek chip (www.pocketlogger.com) would be a much better route, since it doesn't alter the boost sensor signal.

Thanx for the reply Rocket,where you I find a good set of instructions on porting a s5?I know there are alot of threads on it but not all info found on the web is 100% accurate.Thanx Chad(by the way does it make any sense to you that this doesn't occur when the car is warm?)

The threads you will find should point you in the right direction. There are porting templates available from companies like Racing Beat that can help out as well.

Not sure why it would be different if the engine is warm or cold, but you definitely should not be boosting until the engine reaches full operating temp anyhow. However, ambient air temps can affect boost creep too. You'll be more likely to encounter it on a cold night, than on a hot day.