TBI Chevy running rich.

Hi all. I'm at the end of my rope with this truck. I'v been fighting it for 6 months and am all out of ideas so I figured, why not ask the board? Any ideas are welcome. The vehicle is an 89 c1500 a/t with the 305.

So what its doing is it runs fine at idle and anything under about half throttle. Anything more than that at higher rpms it bogs down, sputters, losses speed, all that stuff. It revs ok in neutral but if you hold it wide open you can hear it bogging ant it won't rev much over around 3800. If I unplug one of the fuel injectors, (it doesn't matter which one) it immediately picks up and revs up to the limiter. So far i'v replaced:

ignition module

coil

entire distributor assembly

oxygen sensor (4 times 3 Bosch 1 ntk)

(Edit: forgot I also changed the coolant temp sensor)

and nothing's helped. Any help out there?

Coolant temp sensor.  Pull it and there is a test with a multimeter (i forget the numbers but sure it's online) for it at room temperature and in boiling water.  9 times out of 10 tbi problems are the coolant temp sensor

yup.  I think the FPR is also tied to engine vacuum.  If it's faulty (busted spring) it could be going way overboard with pressure when the vacuum drops.

I forgot the coolant temp sensor, that's been replaced as well. I was leaning towards the fuel pressure regulator only because I'm out of ideas. The weirdest part of it all has to be the fact that im not getting a rich code. So far the only cod it sends is an occasional lean exhaust code 44 ( That is what put me through 4 oxygen sensors).

Then check the circuits related to the O2s

Bad relay for the heater?  Crimped wire? Leak in the exhaust upstream of the sensors?  A leak would cause the exhaust to get diluted with additional oxygen causing the ECM to go pig rich to compensate.

And which coolant temp switch did you replace?  There are two.  One for the gauges and one for the ECM.  The one on the manifold is for the gauges.  There is a second one in the head above the #1 spark plug boot IIRC

I got the one for the ecm. I really didn't think it would help as I could look at the values with my scanner and see it get up to temp, but I changed it anyway. I can see the sensor switching between lean and rich like it should which makes me think the wiring is ok, but I'll ohm it anyway. There aren't any exhaust leaks that I can find.

Is it pouring black smoke out the tailpipe when it cuts out?  If it's rich enough to not rev over 3500, it should burn your eyes horribly.

If you unplug an injector and it works, I would wonder whether there is a power supply problem to the injectors, or a shorted injector pulling more power than the circuit can handle.  Make sure you have a solid 12 volts at the injectors when it's acting up.

Remember that O2 sensors don't tell you rich and lean, only how much oxygen is there. Misfires dump unburned fuel and unutilized air down the tailpipe, which can look lean to an O2 sensor.

Well I just changed both injectors and rebuilt the fuel pressure regulator. No change. I might try swapping the tps sensor tomorrow since I have one laying around, but I don't think thats gonna do much. It doesn't feel like its misfiring, even when its acting up, just bogging down. last week it decided to run fine for a day, I think just to mess with me. The ls drivetrain sitting in the barn is starting to look like the easy button.

You positive you dont have an exhaust leak? That might confuse the O2 sensor/

I can't hear any. I might try some soapy water but if its that small could even cause a problem?

Computer will be ignoring the O2 at heavy throttle.  The sensor is probably HELPING the engine run well at idle and light loads.  I've a feeling if you unplugged the O2, it would be pig rich all the time.

It also might not go over 3800 in neutral at all, if I read you correctly.  In the early 90s, GM started reducing the rev limiter to about that speed in park or neutral.

I had an electrical problem one time that would cause the injectors to overfuel the engine to the point that it just bogged out. I had a wideband on the car that would read down to 9.0 and it would peg 9.0 and then lose power when the issue occurred, so i know it is possible to have a condition that rich, although very rare. I wonder if your flashing injector ground is turning into a continuous ground when the issue is occurring? 

I think there's a MAP sensor on these....my recollection is they can be checked using a voltmeter and a vacuum gauge.  Somewhere there was a youtube video on how to do it.  I went through this a couple years back on a 454 TBI.  Turned out it wasn't rich, it was actually lean- and the problem ended up being a weak fuel pump. 

In reply to volvoclearinghouse :

There is, it's usually on the right side of the engine at the rear just over the where intake and valve cover meet.  Check the vacuum line from the intake for cracks/splits, and there's a couple of 90-degree bends in that line that like to crumble to dust after years of underhood heat.  I'd check fuel pressure too if you can.  Last resort check the ignition module in the distributor and the pick-up coil as well, there are youtube videos out there for all of it. 

I'm going to go put on a weird thing here...

You description was actually making me think you are running lean.

If you have a failing fuel pump (super common for gm, it seems), are you getting enough fuel pressure to work well with both injectors?  Is unplugging 1 of the injectors actually allowing MORE useable fuel into the intake and therefore it runs better?

I had a tbi setup on a 350 in a 78 Malibu.  I had a fuel pump start failing and it was the exact symptoms you describe.

I guess it could be lean. I had that problem about a year ago and replaced the fuel pump with a delphi unit. I should be able to mist some fuel down the Tb and see if it picks up right?

Yeah, if you can get someone to floor it and it only revs to ~3800 and runs badly, you could stand by the fender and shoot some brake cleaner/starting fluid/whatever at the tbi and see if the engine revs up. If so, that's a lean thing. 

In reply to wvumtnbkr :

I was leaning towrds lean, too, but the unplugging the injector thing threw me off.  Your explanation could be plausible. 

Fuel pumps on these trucks are notorious, and it seems the best thing to do is cut a service window in the bed to access them.  Replace the fuel filter annually, and never ever run below a 1/4 tank. 

As you've discovered, there are a bunch of things that can cause the issue you're experiencing.  Rather than firing the parts cannon at it I suggest that you  pick up a used OTC 2000 scan tool off eBay.  They generally go for under $100.00.  That's the tool we used at GM dealerships back in the day.

I'd like to see you check the fuel pressure as well but a gauge with the GM TBI adaptors is a little harder to come by and much more expensive.  It might be worth paying someone to check it or let us know where you're located.  There might be a member nearby who has one you can borrow.

I have a scanner for it. It's an old snap-on brick but it works well enough. Are there any specific values I should look at?

thundrecougar said:

I have a scanner for it. It's an old snap-on brick but it works well enough. Are there any specific values I should look at?

I can't remember if that has 1 or 2 O2 sensors ahead of the cat(s) if it has 2 then it would be good to know if they're reading close to each other.

Does it go into closed loop and does it stay there when it acts up?

Do all of the temperature sensors make sense?

Are you getting good cross counts and are the O2 sensors making sense in the RPM range where it seems to be running well?

Does the TPS make sense?  I don't recall the range but I think it's 0.5v to 5v.  The important thing is that it slowly increases without revisions as you slowly press the gas pedal down.  You can do this with the key on and the engine off.

Does the MAP sensor reading make sense?

As I'm typing this I'm wondering if you don't have jumped timing chain.  Don't forget that a fuel injected engine is still an engine and it needs all the other stuff to work as well.

Maybe some summarizing should happen...

O2 sensors work, cycle, but are not used for feedback at WOT.  

FP failing would be really odd- I don't know of a pump that fails by adding fuel.

Fuel Pressure regulator rebuilt.  if the signal was bad, it would reduce the amount of fuel flow by a likely reduction of pressure.  

coolant temp sensor replaced

ignition all replaced.

Is this MAF or MAP??  (I don't know, wondering)

One thing O2 related- it's *possible* that it's learning an error at low speeds that isn't real.  Say there's an intake leak that isn't getting measured- it's small but contributes at low loads.  In theory, the system could learn a bad lean setting, and correcting rich.  That rich correction would be learned to all times- which *might* make it too rich at WOT.  

The problem with that theory is that the computer has probably been disconnected from power many times, and that learned error has been lost more than once.