Engine Problems...Ignition? Throttle Body?

[Viper_Driver]

I've got an 86 SVO that has developed a few problems which I think may be related. First the gas mileage has dropped from around 21-22 mpg to 15-16 mpg on several tanks of gas. The car doesn't idle quite as well especially when cold (a little rough and the RPMs are a little higher than it used to be). The biggest problem I have is under load and especially above 4K RPM the car bucks, I mean violently bucks, and power out put drops way off. There's no backfire or knocking that I can tell when this happens.

My car has 87k, the problems started soon after I changed out the throttle body for 65 mm using the Tiny Avenger adapter kit and I also changed the stock coil for a MSD TFI coil. I've checked, double checked and tripple checked the TPS off the throttle body and its good. I've run ever EEC diagnositic I can think of and still have no smoking gun. My friend who works at the local shop down the street claims I need an MSD ignition box and the problems will go away, I'm not so sure. We initially thought it might be a fuel issue so I ordered some dodge +50 injectors, they just made the car run worse. So I put the stock injectors back in. I upped the fuel pressure to 50 psi, no change. I've pulled all the plugs and wires they are all good. All the cylinders have good compression.

I'm running a
t3/t4 turbo with a blitz duel selnoid electronic boost controll
65 mm throttle body with T/A adaptor
stock intercooler with bailey BOV
Walbro 255 lph fuel pump with Kirban FPR @ 50 psi
ported / matched intakes
big valve head with stock cam
ported / matched exhaust manifold
3" exhuast
stock ignition except for MSD TFI coil

[SVOeric]

I agree with Mike -- if the problems started when you changed throttlebody, go back to stock, and see what happens.

a couple of quesions..
have you seen the fuel-pressure while under load ? -- I know you have a walbro 255lph pump, but I had a new one as well, and had problems because of a hose in the tank.

Do you still have the wire in the hose between the VAM, and turbo? -- could this hose be collapsing ? (doesn't explain poor fuel economy tho)

have you checked the vam wiring? -- make sure it is in good shape.

What kind of plugs and wires are you running

what voltage are you setting the TPS at?

[stackz]

check the TB gasket and the vacuum lines going to it if there are any. sounds like a vacuum leak to me.

also, check that msd coil out. look for white spots on it or burn marks by the coil wire. It could be breaking down under load in the higher rpms or arcing to ground against itself from a bad coil wire. Bad thing is, I think that unlike the stock coils, the msd coil has a plastic coating around its square magnet so it might be impossible to see if it's internally arcing (usually the white powder will form between the part of the magnet and coil body) hard to see with a plastic coating over it until it finally burns the coating.

did you notice any scarring on the porcelain on any of the plugs??

[SVORob]

I'm with Eric check the wire in the inlet hose. I had that problem it can actually suck the hose pinched. Been there done that lol

[Viper_Driver]

After I initially changed out the throttle body and I started to get the bucking I was assuming I was running out of fuel for the engine. I think its fair estimate that my car is making 280-ish hp and that seams to be the magic number for stock injectors.

I do have the stock tubing between the VAM and the turbo, but it feels like its in good shape. Is there any more scientific way to make sure its good?

I'm running Ford wires and plugs (I can't remember the part # off the top of my head, but they're just stock heat...32C pehaps).

I'll check the coil and plugs, but I don't recall seeing anything unusual when I had it apart just recently.

I have not checked the VAM wiring, but I did run the car which a code reader while driving. I didn't get any codes when the problem occured. I assume I'd get something if the EEC was having problems talking to VAM.

So I guess the consenses is that its not a lack of fuel, if anything its too much fuel. What does an engine do when it runs out of fuel? Is there anything else I can check on the throttle body other than the TPS that could be screwed up? I set the idle voltage to .75V

I'll check out the other stuff and let you know how it turns out. Thanks!

[SVOeric]

ooooo --- .75 is way too low ---

it should be about .91 or so.

[Gator]

i didnt see it.....but did you try swapping out the coil for the old one to see if that cleared anything up...?

I have had MSD coils go bad and cause very bad "studders" at higher RPMs