Any chevy techs out there?

[64fj40]

I need some help with my 96 Suburban. It's a 1/2T 4x4 with the 5700 Vortec. According to the computer, cylinder 3 misfires virtually all the time if not every time (the computer determines mis-fire by noting a momentary loss of RPM and comparing that with engine indications to determine which cylinder is at fault). The problem has been going on for at least a couple months.

I've checked all the wires, replaced the spark plugs (it looked fine) and the cap & rotor. I've run 3 full 42-gallon tanks of gas with injector cleaner through it. All the injector coils tested ok (or at least the same) with my DVOM. I'm thinking that my next step is to replace the injector(s). The problem is that I don't have much experience with newer motors and am hesitant to tear the intake apart without some reassurance that a) I'm doing the right thing, and b) it'll all be ok.

Anybody have any 96-99 Vortec (5700 or 4300 are the same) injector poppets laying around? Maybe an entire spider in a scrapped intake manifold?

Another thought that just occurred to me is that it could be a head gasket failure... How do I check compression on this? I have a compression tester, but I've never needed to use it before...

It's the wife/family rig so I can't have it laid up for weeks while I track down what's going on. Any ideas or help are appreciated.

Eric

[carnuck]

If the motor isn't losing coolant or overheating, you can probably rule out headgasket. Maybe intake leak or 1 side of the throttle body is sucking air (if you can determine which ones miss, and establish a pattern) Stuck EGR (if it has one) will cause this.

[wheeler4life]

ok, maybe this will help. i can't quite picture your injection set-up in my head but if you can to this test it will asnwer some questions. with the engine at an idle can you crack the fuel inlet line at the injector? you should hear a difference in the way the engine runs with the line cracked. you have to open it far enough to have a little fuel leak out. if there is no change then the injector is not firing and contributing. then you need to get a hold of what's called a noid light to plug into the injector wiring harness. if it doesn't flash with the motor running (just like a spark plug would) then it's not getting signal. if it does flash then you know you have a bad injector. again, i don't know if you can access the fuel inlet but if you can that will help. i'll check alldata at work on monday and maybe find some more out for ya.

[64fj40]

The Vortec just has one (or maybe two?) high pressure supply line that feeds a pod of 8 independent injectors. I don't think I want to crack that one while it's running. The only noticeable symptom is a severe vibration harmonic at partial throttle between 2100-2500 rpm which means only on the freeway and only when going up a slight to moderate hill so idling in the driveway doesn't work. Any more throttle, and the tranny kicks down so it's out of the rpm range, and any less throttle, it's smooooth cruisin'.

And the spark plug looks like it should with nice even deposits, so I don't think it's just firing in air. I'll have to check into the noid light, though just to make sure the computer's telling it to work before I rip it all apart.

And it does seem to be losing a LITTLE bit of coolant. Like about a quart over the course of 10K miles.

It's starting to really piss me off and as much as I hate to do it, I may just take it to a dealership and get it fixed for good.

Thanks for the ideas...

Eric

[Crash]

Typical vortec problem. You have a sticky poppet valve and if it doesn't mss continuosly you can have a high pressure clean of the injectors. If its continuose, you will need to replace the injector/poppet assembly. Never seen it happen ;)

[64fj40]

Is it weird that I'm excited about that answer? I'm just happy that I'm hopefully on the right track with this.

Can I just replace the one bad poppet? They're pretty spendy to begin with and 8 would be a LOT more...

Thanks Crash!

Eric

[wheeler4life]

just talked to a co-worker today and he confirms what crash said about them being notorious for poppet problems. no gaurentee's but sounds like you're on the right track. good luck.

[64fj40]

Thanks for all the help. I'll be sure to let you all know how it goes. Or, maybe I'll be back asking for more advice...
At some point I still have to figure out a speed-sensitive vibration that isn't a driveshaft or the tires since they're all new...

Eric

[wheeler4life]

what do you mean by "new" on the driveshaft? the reason i ask is i worked at a driveline shop for many years and possibly could help with your troubleshooting

[magicbrains]

General motors sent out a bulletin with a special high pressure procedure for unsticking and cleaning the poppet style injectors, that would be what Crash was talking about. It usually works. Pouring cleaner in the tank or a standard "fuel injection service" won't help much if they are already stuck. Also there have been problems with the fuel supply lines leaking inside the intake.

[Crash]

Originally posted by magicbrains
General motors sent out a bulletin with a special high pressure procedure for unsticking and cleaning the poppet style injectors, that would be what Crash was talking about. It usually works. Pouring cleaner in the tank or a standard "fuel injection service" won't help much if they are already stuck. Also there have been problems with the fuel supply lines leaking inside the intake.

The procedure requires a special tool, uses air pressure to blow the poppet open, as when they are stuck closed, you cannot get the cleaner thru them therfore cleaning it... Be very carefull with the in tank cleaning solutions, as we have had a # of egr issues due to the in tank stuff..

The supply line issues are on teh early cpi 4.3's, we don't have any with the SCFI motors.

[64fj40]

Ok - so this is sounding more and more like a job I don't really want to do myself - wrenching time is rare enough for the cuiser to spend it on the burban. Should I take it to a dealership (Speedway or other?)? Or is there a specific shop you would recommend?

wheeler4life: I have a speed-sensitive vibration (independent of tranny speed, gear, engine speed, etc.) that appears from 56-65 mph. It's been there since we got the truck a few months ago. Since then I've replaced all the tires with new Goodyears, new RS5000 shocks, and a new rear driveline from Drivelines NW. The tires and shocks were bad and needed to be replaced anyway. From the number and location of the dents on the underside of the body, it looks like someone blew a front ujoint on the rear shaft and it banged all over the place before it got stopped. Someone replaced the ujoints, but managed to pinch both yokes to the point that the ujoints were no longer aligned. Combined with the dents in the tube, I decided to just get a new one made. The vibration problems got a LOT better, but this one persists. I haven't put a lot of thought into it, but I'm starting to think it might be a CV joint up front since the front shaft doesn't spin in 2HI and there aren't many moving parts left...

Eric

[GaryTJ]

[QUOTE]Originally posted by 64fj40
[B]Ok - so this is sounding more and more like a job I don't really want to do myself - wrenching time is rare enough for the cuiser to spend it on the burban. Should I take it to a dealership (Speedway or other?)? Or is there a specific shop you would recommend?........


Easy stuff.. Take the vehicle to an emmision shop and check the exhaust. If the hc's are high and the o2 is low, recheck the ignition. I do not recomend aftermarket caps as they burn through even faster than the oe ones, and they are bad enough.

If the hc's are low and the o2 is high, then you have a fuel problem. The way we clean them is to unlpug the pcv and take a vacuum hose and put it in a can of seafoam and run the motor untill the can is half empty. Shut the motor off and let it sit for 10 minutes. this softens the buildup on the injectors. Start the motor and drive it for about 5 miles to clear all the seafoam out and then with one person holding the throttle @ a fast idle, pinch off the return line to the fuel tank for NO MORE THAN 3 SECONDS.

This spikes the fuel pressure to about 150psi and will blow the crud off of the injector.

GM has a retro kit to replace the cpi injectors with regular MPFI, but it is very expensive.

good luck.. :)