94 Yukon Fuel Starvation - 350

James,

You're forgetting the quintessntial automotive rule. KISS. Keep it simple stupid. Before you start throwing hundreds of dollars of parts at it check for vacuum leaks and scan the PCM for DTC's. If it was a fuel delivery problem she'd backfire and conk out at WOT.

Doc

You're correct. I'll check the computer, what the heck I haven't done that in over a year. I haven't seen a service engine light, but maybe it caught an error that it didn't tell me about.

As for checking vacuum leaks I'll check that out but other then visual inspection is there a good way?

As for WOT, I've actually never tried it. The bucking is fairly violent so I tend to let off as soon as it starts happening. With 130k on all the original running gear I'm afraid to abuse anything. Thanks! -James

spray carb cleaner on all the vacuum hoses... if the idle changes, you've found your leak....

-Bret

James,

Two ways to check for vacuum leaks:

To check the intake and TBI gaskets, spray some carb cleaner around the mating surfaces. If there is a leak it'll suck up the fluid and the idle will momentarily change.

You can also plug the vacuum ports at the TBI, one at a time and take her for a spin. Unfortunately you need to leave the MAP line intact and unless you're insane I'd leave the brake line intact as well! You can plug the EVAP, EGR, and heater control line (if equipped).

Also, pull the plugs and see if they're black (rich) or whitish/orange (lean).

Doc

--------------------- Did you just say "Violent Bucking" ? That sounds more like an ignition thing than your original "surging". Would you discribe it a cut and dry "jerking", kinda like you turned the key off and on very fast randomly. If so have a look at the plug wires, cap and rotor. Look very closely at the inside the coil wire boots. If its black and the tip of the core is gone where it should be bent over and crimped to the metal part, replace it. If you have to, carefully remove the boot on both ends to have a look. No way the O2 will cause a "violent bucking". No there is no float. With 130k on it, it sure wouldnt hurt to put new wires and plugs in it anyway(if you havent allready). Its had plugs put in it before, right? If not, do it now!!! :)

The plug wires are original, they where OK about 10k ago. Did a tune-up about 10k ago, replaced the plugs and pulled the distributor cap off just to have a look and the roater fell apart as soon as the cap came off. So it got a new roater and cap at that time. (probably it's fourth or fifth set of plugs, they're cheap and easy to replace so I do it fairly often, always AC/Delco)

As far as violent bucking, if you've ever ran out of gas in a car when you where on the throttle pretty heavy that's what it's doing. torque goes to zero for half a second or more then comes back for the same period of time, does this over and over again. RPM drops off 1 - 1.5k with the power loss. So it is exactly as if you'd cut all power to the plugs, but also the same as if it suddenly didn't have enough fuel to keep running.

Also I tried WOT on a flat bit of road at about 60mph the other day, the bucking continues for a few seconds then the engine settles down to where it doesn't sound like it's running real well, the tach is vibrating rapidly up and down 100rpm or so and there is WAY less power being produced then should. i.e. it was not accelerating much over 60 even at WOT. This is a

I had ruled out ignition since it starts fairly well and runs OK at low throttle settings, except for the strange "waggling" of the tach.

I wish to heck I could just find a reasonably priced fuel pressure gauge so I could just check the pump and regulator. However it seems impossible to get one for less then $50 even on-line. And so far I've been totally unsuccessful at finding and older style fuel pressure gauge in the 1 - 25psi range. It hard to tell the difference between 9psi and 13psi on a 100psi gauges like they sell today. Not to mention that lots of gauges aren't accurate for the first 20% or so of their range.

Anyway, thanks for the help, and while I'm working on it this weekend I'll check for vacuum leaks, pull a plug or two and see what they look like and check the coil, distributor, etc. I may even just measure the volume the fuel pump will put out too, however I can't seem to find any numbers on much a good pump of this kind should be able to pump. But I did find the fuel pump test jumper so it should be pretty safe to so, as the ignition will be off. -James

James,

Any signs of a head gasket leaking (low coolant, green plugs, green 02 sensor). I'd run a compression test on all eight cylinders ASAP.

Doc

Well I worked on the truck a bit this weekend.

I checked the volume on the fuel pump and it was very good.

Checked for vacuum leaks and found one, the EGR valve had come out. It just pushes into the valve cover so I guess it had just wiggled loose. That's the only one I found.

Checked the plugs and they looked good. No signs of fuel fowling, no oil, no ash. They where pretty clean but warn. Gap should be .035 and it was more like .041 to .045 So I replaced the plugs.

I also took the air filter off and the plastic ring it sits on that goes around the area where the fuel injectors are. It was pretty gummed up in there, lots of that icky brown stuff that comes from the fuel additives. I cleaned all those areas up with carb cleaner. I also fired it up with the air cleaner off and could see both injectors firing just fine. Even at high throttle the spray from both injectors is nice and uniform.

Anyway, checked all the other obvious things, oil is clean, plenty of coolant, no signs of leaks anywhere, air filter is clean, and no codes in the ECM.

So I took it for a test drive, and I was unable to make it do the surge / bucking , jerking thing it was doing before. The tach still wiggles up and down at pretty much any RPM, so at least part of the problem is still there. But hey, maybe it's ignition after all. I had a good look at the plug wires while replacing the plug and they looked good.

So I'm thinking I'll check the timing and maybe think about a new coil. Any thoughts? -James

==================== Did you check the coil wire like I discribed earlier? I have seen the end burn off on quite a few Caprices with the 350CI and a tach and it makes the tach "jump" and makes the car miss badly, but the miss is intermittant.

Well Scott wins the christmas turkey.

The new plugs made the problem better, and new plug wires seem to have fixed the problem all togeather.

No more tach wiggling, and it seemed to start easier also.

Thanks for all the help everyone! -James

P.S. I'm about to post two other messages about other problems with this same truck.

===================== Can we make that the "Christmas Bottle of Jack" ? :) If you have the old coil wire yank the boots off and take a look, I'm very curious.

Wowsers! I dug the plug wires out of the garbage and there where two wires that had one end on each that looked terrible. The rest where clean.

I cut the boots off the ugly ends and took some pictures.

This is a picture of the coil wire (Which looks great) and the ugly ends of the other two wires: (You may have to copy and paste or type the line below into your web browser)

Anyway, thanks everyone for the help. The truck has power again, makes sense since I was probably driving a V6 for the last 10k or so when it should have been a V8.

Just let me know where to send the bottle of Jack, Scott. :) -James

============================ That's what I was looking for. I was expecting you to find the burned ends on the coil wire though. Glad you got it fixed! :)