OT - TBI SUburban troubleshooting

[tjthorson]

Well, the suburban has been acting up for at least a few weeks. Has a very definite stumble at idle and just off-idle. Once moving it seems to be fine, all the power is there and its smooth after about 1500RPM. This is a 94 TBI 350 with 136K on the clock.

It feels like an ignition problem, so I started with the usual, cap, rotor, ignition module for the heck of it. Module looked old, screws were kinda white corroded, heat paste gone, original? GM module.

Mind you, last December, I did plugs, wires, cap, rotor before going to Minnesota. For only a few thousand miles, I did NOT like the look of the cap and rotor. Center on rotor looked like it had 20K on it. Cap was still nice and clean, except the electrodes had a white crust on them too.

My next step I guess is the coil. Unless someone can think of something else I am missing.

It does not do it all the time, mostly when warm. And not everytime you pull away from a light, maybe 2 out of 3 times it does it.

No ECM codes, everything else seems fine....

Coil?

 

[Iceman]

Todd put a scan tool on it and watch the TPS for a dead spot.I just fixed a 95 Chevy P/U with a 350 TB that was doing the same thing.

 

[Axle]

'94 TBI 350, so that would be the LO5, if I'm not mistaken. I've got the smaller LO3 TBI in the wagon and I had this same problem with the car and after a few dead end guesses, it turned out to be that the main PCV hose that goes from the PCV valve in the driver's side valve cover to the front of the throttle body below the air intake box had gotten so clogged with junk that not even brake parts cleaner could push through it. After a little careful prodding with a straightened coat hanger, there were literally chunks of black clay-like crap coming out. I then did the same thing on the breather hose that goes to the passenger side valve cover, and after putting them both back on, new grommets for the breather and PCV valve and the PCV valve itself, the stumble and fumble off the line was gone.

HTH

 

[REDS HOT AIR]

not sure that its the same on yours or not but on my 4.3 tbi the tube coming from the valve cover to the air cleaner spacer blows all the blow by on the fuel pressure regulator and gums it all up

a can of carb cleaner sprayed all up in the regulater will probly help alot dont need to take no lines loose its open on the top side where the spring is

I know I have to clean mine out every couple months since its gotten alot of miles on it

 

[tjthorson]

Well, I checked a few things out. The rubber angle connector coming off the front vacuum port which goes to the PCV on the passenger valve cover was not clogged, but the rubber had split in half on the underside of the piece. I wrapped it tight with electrical tape to seal it up. I couldn't find the regulator area like you said, but I cleaned the whole outside of the thing with carb cleaner. Also hit the injectors and butterflies for the heck of it.

After driving for 10 minutes seems to still be there. SOmetimes it feels like it runs on 7 cylinders. Other times, it idles at a light and pulls away fine.

I thought of another thing, I have never changed the fuel filter - I have put 30K on thing and it looks so rusty (its a GM filter!) it probably has at least 50K on the filter. So, if the weather cooperates I will change that tomorrow.

30,000 miles x 11 mpg average = over 2700 gallons of fuel! Thats a lot of potential dirt in that filter too....

Also looks like the front main seal is leaking.

At least I can throw a water pump in it when I do the seal....

 

[TurboJim]

Originally posted by tjthorson
Well, the suburban has been acting up for at least a few weeks. Has a very definite stumble at idle and just off-idle. Once moving it seems to be fine, all the power is there and its smooth after about 1500RPM. This is a 94 TBI 350 with 136K on the clock.

Pull the vacuum hose off the EGR valve and drive it around, see if its any better. My bet is the spring in the EGR valve is fatigued and its opening too far too early. Very common. Driveability would be stumble or sag just off idle, almost to the point it feels like its choking itself.

Jim

 

[myclone]

Good stuff so far guys.. FWIW TBI systems are VERY sensitive to vacuum signal so make SURE the vacuum lines arent cracked/clogged. Especially the line(s) that feed the signal to the MAP sensor.

Check the TPS with an analog volt/ohm meter and watch for the needle to "flutter" when measuring the resistance in the first 1/3 of its travel (thats where 90% of the driving/wear happens).

Unhooking the EGR is a good idea as a sticking EGR or one with a weak return spring will cause all kinds of drivability issues.

 

[Axle]

When I changed my fuel filter, I didn't feel much difference at all in fuel economy nor performance, however, I'm sure replacing it now saved me from trouble down the road. I don't know how it is on yours, but mine was held onto the frame with this silver bracket that, when first removed, looked to be part of the filter and the replacement one did not have a bracket so I just left it but then it dawned on me, regardless if it looks like stainless steel, it probably rusted, so I had the flat part of the bracket in a vice and I carefully tapped the filter out of the bracket with a hammer and voila!, the fuel filter isn't suspended by the fuel lines.

Also, did you look to see if the PCV valve was functional and that both the breather and the PCV hoses both breathe OK? Now that I'm thinking about it, could it be that the back of the intake box where the passenger side hose connects to in the back is clogged on the box end? I clean that box with simple green religiously from March to November and almost every time I take it off, there's always crap accumulating in there.

Main seal, yup, mine is too, no idea how hard it is, I want to see how it's done and how long it takes before I take it on.

 

[IrvJr]

My 96 Tahoe did the exact same thing and turned out it was the fuel pressure reg. But unfortunately in the Vortechs its under the upper intake, so it leaked right into the rear two runners. I dont think your set up is the same, so nevermind.


Scott

 

[wagon]

What kind of plug wires. I had a similar problem with my 88 TBI. I replaced cap, rotor, coil, plug, and fuel filter, even though they all had only a thousand miles or less on them. I didn't replace the wires, because they were new also. I spent a couple of months trying to figure it out. It turns out that it was the new wires. I pulled one plug wire, and got no change, another and another. I had 3 cylinders that had little or no change when the wire was pulled off. I put on some Delco wires, and haven't had a problem since. The thing is, sometimes it would run bad, and other times it was hard to tell. No stumble. Good luck.

Brian

 

[Axle]

What wires were you running prior to switching to Delco?

 

[wagon]

I don't remember the brand. I bought them at NAPA. Maybe Power Path??

 

[tjthorson]

Mr Testa, automotive genius that he is, hit it on the head.

I unhooked and plugged the EGR vacuum line. Drove it to a wake I had to go to far away. Never missed a beat and drove like a brand new truck. Got a SES light on the highway. Hooked the EGR back up for the return trip home. No SES light, ran like crap again. BINGO.


I'll just get a new valve tomorrow and be done with it.

Might even get a TPS so I don't have to worry about that again either.....

Thanks so much guys!

 

[tom h]

Originally posted by wagon
What kind of plug wires. I had a similar problem with my 88 TBI. I replaced cap, rotor, coil, plug, and fuel filter, even though they all had only a thousand miles or less on them. I didn't replace the wires, because they were new also. I spent a couple of months trying to figure it out. It turns out that it was the new wires. ... I put on some Delco wires, and haven't had a problem since. The thing is, sometimes it would run bad, and other times it was hard to tell. No stumble. Good luck.Brian

Same here, I posted:

Posted by tom h on November 18th, 2003 06:20 AM:

I'm probably in the minority, but over the years, on a variety of vehicles, I've always been disappointed in quality & reliability of non-GM aftermarket ignition parts .... and will only use original GM.

most recent example, on my 94 5.7L V8 big Blazer last year, spent 2 annoying months tracking down an intermittent surging/bucking , that would only cccur after the truck had been on the freeway for 10-15 minutes.

Turned out to be the 10 month old Accel brand plug wires. Funny thing is, there hadn't anything really wrong with the 8 yr old GM wires, I had just felt replacing them might be a "good idea" after 8 yrs. All problems cleared up with the new GM plug wires.

I can sympathize with Todd Thorson. before finding the faulty plug wires, I changed all sorts of parts attemtping to fix an intermittent hesitation & "bucking" problem on my 135K mile Blazer a couple years ago:
-- fuel pump & filter.
-- distributor cap, rotor, ignition module, coil, plugs.
-- TPS.
-- EGR valve.
-- found some cracked vacuum tubing & replaced ALL tubing, even tubing that looked "OK" at the time.
-- IAC valve.

So now, I figure the vehicle should be in good tune for another 100K miles!

 

[TurboJim]

Originally posted by tjthorson

I unhooked and plugged the EGR vacuum line. Drove it to a wake I had to go to far away. Never missed a beat and drove like a brand new truck. Got a SES light on the highway. Hooked the EGR back up for the return trip home. No SES light, ran like crap again. BINGO.

Thats because GM''s EGR "test" consists of watching BLM while on a long decel.

Do youself a favor and get a geniune AC Delco valve and NOT one of those universal ones with the selected orifice. You'll need the number off the old one. Its stamped into the top of it and will be a xxxxxx N or xxxxx p one. N for negative backpressure and P for positive backpressure

If you cant find one, let me know the part number on your old one and I'll get one for you. My shop is an AC DElco repair center and I have access to every part they make and still have available.

 

[myclone]

Originally posted by tom h
Same here, I posted: I can sympathize with Todd Thorson. before finding the faulty plug wires, I changed all sorts of parts attemtping to fix an intermittent hesitation & "bucking" problem on my 135K mile Blazer a couple years ago:
-- fuel pump & filter.
-- distributor cap, rotor, ignition module, coil, plugs.
-- TPS.
-- EGR valve.
-- found some cracked vacuum tubing & replaced ALL tubing, even tubing that looked "OK" at the time.
-- IAC valve.

So now, I figure the vehicle should be in good tune for another 100K miles!

Im with you guys on aftermarket parts.. Sigh... I do a lot of work on neighbors/close friends GM autos and I have a pretty good sized box of Advanced Auto and Autozone replacement parts that Ive removed. 95% of them were new, or close to it, and caused problems for one reason or another.

The point Im trying to make is that when it comes to ign parts or sensors of any kind I use Delco exclusively even if the owner of the vehicle complains about the price they pay for them. I wont waste my time or theirs installing Exact (or similar el cheapo brand) plug wires or some kinda Hop Sing brand sensor on any car.

BTW, Ive used Accell plug wire(s)...yes sir, Ive got one on the riding mower since some mice decided to eat the original plug wire and I didnt have a replacement handy. That box of "take off parts" I mentioned earlier is good for something after all .