I have a 2.8L, bored .20 over, Hypertech chip, 160deg thermostat, cam, milled heads, etc. Cold motor has a compression of 10.2:1; This just started happening in the past week with the weather getting cold here in Texas. As I accelerate, about midrange (2500-3000 rpms) the car starts stuttering very badly and backfiring/knocking. Once I get to high end of rpms this goes away. Could this be the 160deg thermostat screwing up(outside air temperature is about 50-60 degrees)? Any ideas? I just replaced my EGR valve, idle air control valve, and all fuses/wires in the fuel system have good connections. Plugs/wires are new as well. Thanks!

 

I doubt the 160* T-stat is the problem in itself. But we could use a little more info, like number of miles since the rebuild, what was done, fuel injected or carb'd, the base ignition timing, etc.

 

it's the '85 motor, fuel injected; all new bosch injectors (stock); external accel fuel pump, TPS is set at .55 volts, rebuild was done 7,000 miles ago; base idle i believe is set at 800, i'd have to look it back up in the book; the cam and lifters are a crane cam .423@valve; k&n filters, accel plugs/wires, new distributor, accel supercoil on the ignition; remanufactured crank, milled heads; exhaust: dynomax hi-flo cat, 2 1/2" pipes to a flomaster muffler; thanks for the interest in helping! any other info you need just ask.

 

If you had a scan tool, this would help in tracking down a sensor problem. A flakey sensor can be the source of many erratic behaviours. I would seriously consider getting one as the cost of tracking down a faulty sensor can be expensive.

If you have a concern about the 160* T-stat, try putting your old T-stat in and see if it cures the problem.

Since I doubht you have one or acces to one, try checking your plugs. These will give a good indicator if you have something electrical. Make sure that none looked carboned up or oily.

Though you didn't mention it, I will assume that you have replaced your entire timing set when you replaced the cam.

Sorry, but given the mods you made, I don't think you are going to have an easy fix, short of getting a tank of bad gas.

 

Thanks a lot! I'm gonna try to get my hands on one. Yes I did change my whole timing set. Along with pistons/rings, the usual rebuild. This motor has been a pain in the *** to get right because we replaced my carburated '84 with this fuel injected '85 and had to wire the entire car ourselves. Thanks for the help!!!!

 

What about the chip? Did you get a new chip burnt to take advantage of your mods? Based on your mods, I wouldn't expect anything short of a custom chip to work properly with it.

 

well we actually just got the hypertech/160 deg thermo; you recommend a custom to work with the cam, etc?

 

Very much so. Those "off-the-shelf" chips are designed for minor mods (at best). Generally, they play with the locking of the TC and ignition advance...not much else.

Your mods are quite extensive. I suspect that your are probably leaning out at the mid-range. If you have an AFPR (or fashion one), I think that if you cranked up the fuel pressure a bit that you'll find the "shuttering" goes away and your performance increases. It will be a temporary fix, but should help while you get a custom prom.

Also, relocating the MAT/IAT into the intake BEFORE the TB will detect the cooler incoming air and give a slight increase in the pulse width during WOT.

Because your ECM will run in closed loop during part throttle, your ECM will do a lot to compensate for the increased air flow of your engine. It is during WOT when the ECM goes in open loop (actually WOT mode) where it doesn't look at the O2 sensor that you must rely on the PROM (combined with the fuel pressure).

But what you described definitely sounds like a lean condition during WOT.

 

Yeah, one thing I have noticed after this cam install is it either wants me to keep my foot off the pedal totally, or all out floor it. During midrange it's always got a shudder to it even before this sever **** started happening. I'm gonna discuss this with my dad(being that I'm a poor college student and need some financial assistance from the 'rents) and hopefully get a custom EPROM burned. I've had my complaints about the computer anyway because I believe it's what is holding me back. For having as high a compression as I do, and as many mods as I do, I'm just not getting the umph that I expected even though it's a 6. Thanks for all the assistance!

 

Also consider the Adjustable Fuel Pressure Regulator. It really is a great tuning tool to get the optimum performance AFTER you get the proper PROM. But I think it may help out in the short term while you are saving up for a custom PROM.

Also, did you upgrade your injectors to a larger size so it can feed that hopped up V6?

This is what up TPI guys have to do to our cars when we make similar mods to our cars.

 

yes, but not by much; stock is 25lb, we've got 27lb'ers; it's incredibly difficult to find anything for this particular engine. GM has discontinued 75% of parts for it while aftermarket ignores it.