1990 C1500 rough idle/miss

Hi folks.

I've got a 1990 C1500 with a Goodwrench motor (1,500 miles) and a Bowtie Overdrives "Stage 2" 700R4 (450 miles).

Shortly after the transmission install, I noticed a very rough idle and distinct miss during idle. At speed, especially WOT, a vibration could be felt. I was initially concerned the vibration was from the transmission, but I am now able to re-create it in neutral/park. I'm aware the torque converter (new) could also cause the vibration, but strongly suspect the engine.

I replaced the cap/rotor/wires/plugs last weekend. The wires were recycled from the old motor and highly suspect. The plugs were all carboned (note 1,500 miles on factory motor and new plugs).

This really seemed to help everything. A few days later, the truck started stalling - especially after being held at throttle and then let off (ie, approaching a traffic light). I removed the air cleaner assembly and noticed the fuel injectors were practically dripping fuel.

Later in the day, the truck flat-out refused to idle. 1,500 RPM without load was OK, but it would immediately die below that. I suspect the engine was drowning in fuel.

I replaced the injectors and rebuilt the fuel pressure regulator. I removed the TBI, then inspected and cleaned all passages. Everything went together and the truck was almost "like new" for a day. I still had an intermittent "stumble" at idle, but it was remarkably better.

Since then, I've had a couple of trouble codes - sometimes 33, sometimes 34 and sometimes both. I spent the entire morning attempting to re-create these codes and checked every component of the emissions system per the instructions in my Haynes book. All input voltages to the sensors from the ECM were within specification, as well as output. Specifically:

TPS - 5.010 v input. .06020v throttle closed, 4.3v WOT (smooth direct relationship between throttle position and voltage).

Coolant temperatore (ECT) - 5.010v at sensor. 550 ohms resistance with the engine in the 130 degree neighborhood (had been sitting for a while after running).

Oxygen sensor - .397v.

MAP sensor - power from ECM: 5.010v. 1.1v-1.2v engine running.

TPS sensor was inspected earlier in the week, appears to function properly (idle becomes high when disconnected) and had proper resistance per "the book".

EGR valve: Confirmed proper operation with a vacuum pump - I noticed an even more horrid idle with vacuum on the EGR. Releasing vacuum caused the "normal" idle to return.

ESC: The Haynes book doesn't describe it's function or testing. I'm not too sure on this one - I was able to disconnect it with the engine running and didn't notice any difference. What does this thing do?

A gauge connected to the EGR solenoid indicated 18-20 inches of mercury for a split second when the throttle was opened. I know the EGR should open at highway speeds, but is it supposed to open momentarily when the throttle is snapped (ie idle to 1500 RPM)?

This morning I noticed the injectors aren't spraying right. I have streams of fuel again and an occasional "drip". Fuel is puddling up and some of the spray is actually landing on the TBI itself.

At this point, I think most of my issues are TBI related and not ignition.

I can't believe these injectors are bad, but they sure look like it. Unless there's something sensor-related that's causing the ECM to want to run amazingly rich?

I'm running out of options here.. does anyone have any ideas?

-Steve

Code 43 is directly related to the ESC and the knock sensor. If you were able to disconnect the ESC while the engine is running, and no change was noticed, there is definately a problem. You should be able to hook a timing light to vehicle and while the engine is running, unplug ESC, ignition timing should retard itself and increase in idle should be noticed. ESC has the following connections:

1. 12 volts
2. Ground
3. Direct connection to knock sensor (5v)
4. ECM signal wire (probably pin B7 on ECM 8 - 10 volts)

Check connections @ Knock sensor, this connection is easily damaged or contaminated. Make sure wire from ESC to ECM is not open or shorted. Verify voltage and ground at ESC and ECM.

If code 43 is set hard the timing will always be retarted and your vehicle will act retarded.

Hope this helps!!!

Zac Mowat

I saw code 34 in your statement and read 43 in my mind, is 43 showing by chance??

Hi Steve,

Well, I just had my week from hell with my C1500.

This might be what you are seeing, hopefully.

My 4.3L starts fine. idles up. 20 seconds later,
when cold, idles down, tries to recover then
dies.

When it warms up around 200F,
it would run better. still crappy.

Loooooong story shorter:

i changed a lot of stuff!!

the coolent temperature sensor doing its job,
goes closer to zero ohms, (cause it ran pretty
hot at highway speeds --my bad--).

All i can figure is it damaged a current limiting
resistor in the computer suppling 5 volts
to the coolant temp sensor.

at 500 ohms around say ?? i dont know, 150 degrees F.
that would be normal for the computer.

well, if the resistor is damaged or something else
is, in the computer, its not going to know
the correct temperature.

So, my first clue to this was a brand new sensor.
Stone cold. it was 20 degrees yesterday.
so you know how happy i am to get this fixed.

I plugged it in and laid it on top of the block.

OH BABY! It just run perfect. I shut it off
and restarted, and again, just perfect.

So, i stuck it into the block and the truck just
run like crap again.

What in the world??????
(#%^&%^###%^% -- i was sad.)

So after vacuum and soul searching and sleeping and eating on this, I wound up some resistors
with some values on the temp. chart from
the chilton cheapie book, around 10,000
ohms. I was going to fake out the ECM and
see if my guess was right: Does it think it's too
warm out? And sure enough the old girl just
run smooth and too fast, but run !!!

So, looking at the chart, kept lowing values down,
and the idle would also lower. And DID NOT
STALL.

So, I plugged in the OLD stone cold sensor.
and she run just fine.

So it seems to me, the computer line for that
device is bad. out of calibration.

Now! The sensor wire tied to the map
and knock sensor plate above the valve cover
on the right side of the engine.

took a test drive and got back ok!!

It is my belief, letting my engine get so hot,
not changing the thermostat when i should have
made the coolent sensor do its job, which in turn
exposed some bad engineering on behalf of the
ecm maker, and gave me all kinds of trouble in
cold weather.

I hope this solves or gives you clues!!

Gregg