Need Help with a TBI350 Please!

[Dylan Barnesy]

Im going to make this as short as possible, however how my truck is acting takes some describing....

First off its a 1995 TBI350 with a NV3500 (originally an auto truck), 1/2ton, 4in suspension lift, 3in body lift, on 35's. Stock engine with headers and exhaust.

So when the truck hasnt been ran in lets say 6+ hours, it will start up perfect, and run perfect, zero issues. I can shut it off, start it right back up (sometimes starts, sometimes wont unless I hold gas to the floor then takes right off), and it smells like its running rich, falls on its face, stumbles, also idles high after trying to rev it slowly and VERY slowly idles back down etc. Only way to take off is to barely ease into the throttle and slip the clutch. NOW once you get it going and get into say 3rd gear midway through the rpms and smash on it, it will hesitate and stumble a little but then take off running great. After that happens and you slow to a stop, the idle Slowly idles down, and then bam it runs perfect again. It has done this like clockwork for the past 6+months and im at a loss with it. Ive owned the truck for approximately 4 years and these issues started about 6+ months ago, Ive always been a carb guy so have just never tackled the issue since it is still "drivable" and it isnt my daily. But im gettin tired of it and wanting to get the issue fixed but have no idea where to start. Any help would be greatly appreciated!! Thank you!

 

[Dravec]

Is the check engine light on?

 

[LC2NLS6]

Sounds like low fuel pressure. First thing I look for. Narrow it down after that. Rent a free tool at autozone advance etc, install it at the fuel filter is easiest on my truck.

 

[1997]

Is the check engine light on?

^ this, with more details;

95 is OBD1 so codes can be set without the check engine light coming on.
You should either get an OBD1 code reader or google how to get OBD1 codes using a paper clip.

 

[chevman_88]

Sounds like a fuel issue. TBIs are notorious for the FPR diaphragm wearing out or just tearing.

Pull the air cleaner assembly off (including the plastic collar if you still have it) and look at your injectors. Have someone available to sit in the driver seat to help out
1. Engine off and cold - leaking?
2. Engine on and cold - How does the spray pattern look? Nice fine mist / \ shape? Any dripples or dirty spray patter?
3. Shut engine off while cold - Injectors leaking/dribbling at all?
4. Restart engine and let it warm up - Hard start? What did your injector spray pattern look like?
5. Go through steps 2 and 3 with a warm engine.
6. As mentioned above rent a fuel pressure gauge from your autoparts store of choice. These like 9-12 PSI.

Do you know when your TBI was rebuilt last? The injectors have little filter screens on them that will get gummed up with this crappy fuel we have now days. It may be time to rebuilt it if it hasn't been done in a while and make sure to clean all of passages underneath of carbon and crap. Look at the 'ultimate tbi mods' while you are in there and decide if you want to do it (I didn't notice much of a difference for mine, I think it's like the performance benefit of doing the air cleaner lid flip - none). These injectors aren't failure prone like the vortec OE injectors, but there's only 2, so when something goes wrong, you will know it.

 

[thinger2]

Yep, when diagnosing a tbi, always start looking for codes. Under the right circumstance, they can set codes and not light up your check engine
Step 2 is fuel pressure.
You can get a fuel pressure gauge as a loaner tool from the part stores and pick up a fuel filter while you are at it.
These trucks dont have a test port in the fuel system so you "tee" the gauge into the fuel line at the engine side of the filter.
The are about the easiest fuel filter ever to replace and if you havent put one on recently, it needs it.
When you do the injector spray tests mentioned above, do the running test by hooking up a timing light in the usual way and aim it the injectors as they spray. Watching them while the light strobes will let you see patterns your eye cant detect.
You shouldnt see any dribbling, just a spray
When you say you need to push on the gas pedal to start it. That is an air/fuel ratio problem. You should never need to use the gas pedal to start a fuel injected engine.
By pushing on the pedal, you are changeing the A/F ratio instead of the idle air control valve controlling the A/F ratio. Flooring the pedal puts it into "flood mode"
Flood mode is a situation where the ecm stops the pulse to the injectors and tries to fire off of too much fuel.
In other words, the ecm is trying to set the A/F but it is being overridden by you putting into flood mode


Usually, a high idle is caused by a vacuum leak or a stuck throttle/ stuck linkage.
You might also have a sticking egr valve.
The good news is that all of this is pretty dang easy to fix and parts are dirt cheap.
So look for codes first, all you need is a paper clip to check for codes on an OBD1 its easy
Then, if you have no codes, check fuel pressure, should be 9 to 13 pounds but if its good 11 or so seems to be normal.
If all of that checks out okay, Now take a look at base timing. You already have the light hooked up right?
Base timing needs to be at 0 degrees with the timing reference wire disconnected.
On a 95 that is a tan wire with a black stripe that hangs out under your glovebox next to the blower moter.
Is has a plastic disconnect.
Do this with the engine warm.
Once you have fuel pressure, base timing and good injector spray patterns
The rest is cake.
Check it out, let us all know what happens.
And, pay close attention and tell us how it behaves at cold start and what is different at hot start
I think you have a couple of things happening here.
But, it aint rocket surgery brother, its just a pain in the abs from leaning over the damn thing
Check it out and post what you find
Next, if all the fuel part of this is good, and you have completely amd beyond a doubt searched for vacuum leaks.
Beyond any doubt!!!
Then its time to take a look at the EGR valve.
More cake .
When its stone cold, disconnected the vacuum line, stick your fingers into the egr valve and try to push it towards the firewall.
It should push all the way in.
Then, it should release back towards the front of the truck.
If it sticks all the way when you push it in.
And it wont start,
Wallop the bejeezus out of the EGR with a rubber hammer or a deadblow.
The best way to do this is to pull the vacuum tube off of the EGR solenoid and put no more than 15 inches of vacuum and see if it holds.
A stuck EGR wont pull any vacuum at say 8 or 10 inches but it can "Pop Loose" .
My fellow friends. You Gmt400 crazy people.
Get yerselves a "mighty vac"
And a rubber hammer
40 bucks.
Just dont whack the damn thing wifh a rubber hammer when its screaming hot.
Safety First you nut jobs

 

[Schurkey]

Lots has been said about "codes".

Sure, codes can be useful.

I have more faith in the data stream. For this particular truck, if the fuel pressure checks out, and the distributor cap/rotor/plug wires/plugs aren't ancient, and the coil will reliably fire a spark-tester calibrated for HEI ignitions, I'd want to know what the coolant temp sensor and it's wire harness is telling the ECM. I'd want to know what the O2 sensor and harness is providing, and how many cross-counts the thing has at, for instance, a 25 mph cruise vs. idle in gear while stopped.

Connect a REAL scan tool, (not a "code reader") let the ECM tell you what's wrong.

 

[thinger2]

As a matter of fact I find that codes are quite often usefull
I actually make it a habit to try to dig on down way way deep when the flashy failure lights start blinking.
Hell, I even carry two paperclips.. gotta have a backup
Just cuz I wanna kinda see why the flashy lights blinking.
Codes are kinda usefull

Check for codes, check fuel pressure, check for fuel pattern.
Coil, cap rotor, plugs, wires.. etc But, i bet youve already done that right?
Live data scanning Is great after you Check the basics
Start with the basic well known and readilly available GM diagnostic flow chart process.
Fuel pressure, timing, injector pattern and pulse.
Or, skip all of that and chase your tail for awhile.
It really depends on your level of experiance too.
Connecting a "real scanner" wont help you a whole lot if you dont know what you are looking at.
So the truck sits for a month while you do a crash course on OBD diagnostics and it turns out to be a bad fuel pump?
Nonsense
Start at step one.
Dont let people try to talk you into making this overly complicated.
It really isnt.
At some point you might need a scanner and live data.
But you aint there yet. Youre not even close.
GM TBI is the most knucklehead simple computerized injection system ever devised. It just takes a little getting used to but its basically a carb that uses sensors for adjustment instead of the user randomly turning screws to make it run.
No moon launch involved brother.
Start at step one, cross that off the list and go to step two....etc
Do it in a systematic way and you will be done and it will be fixed

 

[Dylan Barnesy]

I appreciate all the replies, ill check into all that is mentioned when I get the free time and let you know what comes of it.

 

[Rico387]

Im going to make this as short as possible, however how my truck is acting takes some describing....

First off its a 1995 TBI350 with a NV3500 (originally an auto truck), 1/2ton, 4in suspension lift, 3in body lift, on 35's. Stock engine with headers and exhaust.

So when the truck hasnt been ran in lets say 6+ hours, it will start up perfect, and run perfect, zero issues. I can shut it off, start it right back up (sometimes starts, sometimes wont unless I hold gas to the floor then takes right off), and it smells like its running rich, falls on its face, stumbles, also idles high after trying to rev it slowly and VERY slowly idles back down etc. Only way to take off is to barely ease into the throttle and slip the clutch. NOW once you get it going and get into say 3rd gear midway through the rpms and smash on it, it will hesitate and stumble a little but then take off running great. After that happens and you slow to a stop, the idle Slowly idles down, and then bam it runs perfect again. It has done this like clockwork for the past 6+months and im at a loss with it. Ive owned the truck for approximately 4 years and these issues started about 6+ months ago, Ive always been a carb guy so have just never tackled the issue since it is still "drivable" and it isnt my daily. But im gettin tired of it and wanting to get the issue fixed but have no idea where to start. Any help would be greatly appreciated!! Thank you!

I just went through the exact symptoms you are having. It was the Throttle Position Sensor.