TBI 454 lacks power

[Dr_Snooz]

Hi guys,

I've been fighting this for awhile with no real luck. The newly rebuilt 454 in my 90 1-ton TBI lacks power. The idle surges and all the power evaporates at mid-throttle. It struggles up hills and has been slowly getting worse for the last few weeks.

I've replaced the cap and rotor. Plugs and wires are relatively new. Coil tested fine. Pickup coil tested fine. Timing is dead on 4 deg. BTDC with the EST in bypass mode. Fuel pressure is 13 psi at idle and the fuel pressure regulator is new. Fuel filter is also fairly new and the fuel injector pattern is not noticeably bad. I re-gapped the plugs when I installed the new engine ~2,000 miles ago, though I haven't checked them recently.

This sucks and I'm out of ideas at this point. Can anyone help?

[Dr_Snooz]

Oh yeah, the ICM is also new after the old one left me stranded a few weeks ago.

[Dr_Snooz]

I tested the TPS today. It's supposed to get a 5V supply. Mine is getting 0.818V. Hmmm. I assumed the ECM would flash a CEL if I had sensor problems, but I guess not. The "Service Engine Soon" light does come on when I turn on the key and turn off a couple seconds later, so it does work. According to my Chilton's manual, I have either a problem with the wiring to the TPS or a problem with the ECM. I dug into my GM manual to verify that, but of course, that section is in a manual I don't have. Grrr. I am in the process of downloading the GM manual now and will dive into it later.

When I had the engine out I took time to rewrap my engine loom, which had fallen apart. I'm a pretty careful guy, and don't often screw stuff up without knowing that I've done so, but it's possible I broke a wire. Of course if I had, I'd be getting zero volts to that TPS. So I'm leaning toward the ECM. Is it common for GM ECMs to go bad? Is there a way for me to test mine at home? Are they expensive? Does anyone have a pinout from the ECM so I can run a jumper wire to my TPS and verify that the wiring isn't bad?

Thanks.

[Irish_Alley]

think the tps is supposed to be .5v at idle and 5v at wot

http://forum.73-87chevytrucks.com/smforum/index.php?topic=29996.0

[Dr_Snooz]

Yes, across terminals B and C I should be getting 0.5V at idle. Across terminals A and B, however, I'm supposed to be getting 5V constant and I'm only getting 0.818V.

Did some reading last night and I guess those ECMs are pretty notorious for going bad. They aren't terribly expensive on Rock Auto (<$200). I downloaded the "Light Duty Truck Fuel and Emissions Manual" which covers all the engine electronics and the first thing they have you check is the ECM. So I'm leaning even more toward the ECM now.

There are some basic diagnostics for the ECM, which I'll perform and see what I learn. I notice that the new ECMs are all reman units, which gives me the creeps, frankly. I've never found reman electronics to live very long. They also require me to reuse my PROM unit, and I'm wondering if the PROMs also go bad?

Incidentally, this link contains a trove of GM manuals in *.pdf form:

http://67-72chevytrucks.com/vboard/showthread.php?t=545416

I'm trying to find the location of my ECM right now and some more description on my ALDL link, what it looks like, where to find it, pinouts, etc.

[bd]

TPS pin A should be signal return (ground), C should be 5-volt reference, and B should be TPS signal (~0.6-5v dependent on throttle opening).  So, with the ignition ON, engine OFF, throttle closed to idle, your DVM should measure: zero volt A-to-ground, ~5 volts C-to-A, ~0.6 volt B-to-A. 

Actually, the ECMs on these trucks are pretty durable.  Reverify the 5v reference, check all ECM grounds and the ECM pin connections, before condemning the ECM.  A floating ECM ground may not set a diagnostic trouble code.

Have you test driven it with a fuel pressure gauge connected while it is acting up?

[Dr_Snooz]

TPS pin A should be signal return (ground), C should be 5-volt reference, and B should be TPS signal (~0.6-5v dependent on throttle opening).  So, with the ignition ON, engine OFF, throttle closed to idle, your DVM should measure: zero volt A-to-ground, ~5 volts C-to-A, ~0.6 volt B-to-A. 

Okay. Well, I know Chilton isn't the final word on these things, but they're telling me that the 5V should be across A and B. With terminal B as ground. Autozone is saying the same thing (http://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0996b43f80c90e96). GM's manual is completely useless on this topic. I'll have to go out tomorrow and see what's what. FWIW, I did poke around on the other wires on the TPS and never got any kind of voltages that made sense. There's something squirrely going on there for sure.

check all ECM grounds

I'll go looking for those, unless you want to tell me where they are...?

Have you test driven it with a fuel pressure gauge connected while it is acting up?

I haven't because my super duper Harbor Freight gauge doesn't seal very well. It drips gas everywhere. If it helps, I did gun the engine and the pressure didn't flicker.

Tonight I went out and did the Tap Test (http://www.napaechlin.com/GM-Tap+Test/Content.aspx) and the ECM passed. I basically just beat on the case with a ratchet. If the ECM is bad, it's supposed to make the truck hiccup. That isn't the kind of test that inspires my confidence frankly. With a German or Japanese car, the tests are very specific and detailed. With GM, they are like "beat on it with a tool or something." Seriously?

At any rate, the SES bulb lights when I turn the key on, flickers once and stays steady until startup. So that's also a pass. I put the ECM in diagnostic mode and it flashed a code 12, which is another pass.

Without verifying the TPS pinout, I'm still leaning toward the ECM at this point. At the very least, the ECM should flash a trouble code if the TPS is sending out bad voltage readings.

[Irish_Alley]

you can always do the ohms test. 5v wont be on a and b if your not opening the throttle 100% (WOT). if you only got .8 at wide open throttle then you would return to idle when you go wide open. if you turn the tps the volts should gradually increase with no dead spots. if you hit .8 while at 100% then that would be a dead spot
T.P.S.
(Throttle Position Sensor)

Symptoms of a Defective TPS

The 'Check Engine' Light is On
Bucking and Jerking/Hesitation while Accelerating
Idle Surging
Sudden Stalling of the Engine
Sudden Surge in Speed While Driving on the Highway

Testing
Test between blue and black wire.
.5v idle to 5v WOT
the readings should gradualy change hitting no dead spots



Or you can test with a ohms meter
TPS Actuator Arm at rest:       Pins               Values (ohms)
                                               A to C       =       5.88k
                                               A to B       =       2.44k
                                               C to B       =       8.32k
                         
Rotating the TPS Actuator Arm:                       At Rest      Rotated
                                               A to B       =       2.44k       8.26k
                                               C to B       =       8.32k       2.39k

[bd]

Since there are contradictions between references, if the TPS wire colors on your engine are gray, black and dark blue: gray should be +5 volt reference, black should be sensor ground and blue should be the TPS signal to the ECM.  Testing as before, black-to-ground should measure zero volt, gray-to-black should measure +5v, and blue-to-black should measure ~0.6-5v depending on throttle opening from idle to WOT, respectively.

[Dr_Snooz]

Thanks guys.

My wire colors are:
  A=black
  B=blue
  C=gray

Terminal C does supply 5V to the sensor, so Chilton and Autozone get the big raspberry blown at them for being lame. Thanks for clarifying that bd. I would have been chasing my tail for years trying to figure that out.

I did some more testing with my multimeter today and didn't get much good.

With ignition on, I got the following readings between terminals.
  A->C  5V
  A->B  3mV and declining
  B->C  0V
Okay...?

With the engine running, I got
  A->C  60mV and declining
  A->B  0V
  B->C  0V
Huh?

With the key off, I checked continuity to a known ground (the battery negative terminal).
  A= no continuity
  B= ~1 kOhm
  C= ~1 kOhm
Not sure what that tells me. I was just looking for the ground wire and not finding it. Without power to the ECM, I guess terminal A doesn't go to ground.
 
I need to think this all over and take a fresh run at it tomorrow now that I know what terminals are what. Thanks Irish for posting that info. I need to test the sensor itself to make sure its output is correct and your info will help a lot.
Thanks bd for keeping me out of the weeds where Chilton had steered me.

[Dr_Snooz]

Those figures are no good. I backprobed for them and it didn't give good results. Today, I pierced the wire insulation and got better numbers.

With key on:
  A->C = 4.99V  (s/b 5V)
  A->B = 0.832V (s/b 0.5V-1.2V) increasing smoothly to 4.3V (s/b 5V) at WOT
So if my Chilton's manual is correct, the TPS and circuitry is working within tolerance.

I also tested the resistance with the TPS unplugged.
  A->C = 5.04kOhms  (s/b 5.88)
  A->B = 2.14kO increasing to 5.74kO  (s/b 2.44kO -> 8.26kO)
  B->C = 5.52kO decreasing to 1.94kO  (s/b 8.32kO -> 2.39kO)
I'm a little outside those ranges and am not sure if that warrants a replacement or not. My only symptoms are a surging idle and no power after pressing the throttle past a certain point. The TPS is $100, so I'd like to make double certain it's bad before buying a new one.

[bd]

Disregard TPS resistance measurements - they aren't reliable.  The voltage measurements you posted are within the acceptable range.  As long as the voltage transition across A/B is uniform/controlled/consistent, the TPS does not need replacement.  However, you might perform a "wiggle & gentle tug test" on the wires connecting to the TPS and MAP sensors with the engine idling

Buy/rent/borrow a fuel gauge that doesn't leak and has the correct adapters, then monitor your fuel pressure while driving under load.  Check the exhaust for restrictions (best accomplished using a pressure gauge plumbed into an air injection or O₂ sensor port, etc).

[Dr_Snooz]

But/rent/borrow a fuel gauge that doesn't leak and has the correct adapters, then monitor your fuel pressure while driving under load.

Yeah. Before it leaves me stranded, right?

I did spring for the much more expensive kit that alleges to work on TBI. It just doesn't seal very well and requires the fuel filter to be removed before using. Thus, it's no help in diagnosing a plugged fuel filter. I'm tempted to plumb a fitting into the fuel line that allows one to use a regular gauge without some leaky adapter.

[Dr_Snooz]

Will something like this work?

http://www.ebay.com/itm/TBI-Throttle-Body-Injection-Fuel-Pressure-Gauge-Adapter-/260732758647?hash=item3cb4e22a77&vxp=mtr

[bd]

Haven't used that adapter, but it appears workable.  You can check the fuel filter by draining it and then blowing through it.  If there's any restriction, replace it.