TBI problems, running rich

I'm working on a 1991 LD2500 with a 350. Has ran rough for several years. Surges at idle, sometimes dies.

Spark plugs are black and sooty. Temp sensor, map, and tps all appear functional, checked with scan tool. BLM is down around 100. Can't see any leaks around the injectors with the key on engine off. Fuel pressure checked at 12psi.

Any help would be appreciated, I'm running out of ideas here.

What is O2 sensor voltage, and O2 crosscounts?

Is the engine running in closed loop, or in open loop once warmed-up?

Does the spark advance work? How old are the cap, rotor, and plug wires?

A very-lean Block Learn indicates either a faulty O2 sensor, or excess fuel getting into the intake manifold (which is somewhat unlikely if the coolant sensor and MAP are working properly)

Is the outlet of the catalytic converter hotter than the inlet?

O2 sensor does switch between high/low. I'll have to check what the values are.

It does go into closed loop.

I checked that timing was at 0* with the connection on the firewall unplugged. It advances with the wire plugged in. I didn't check how many degrees. Cap, rotor and wired are maybe a couple years old. Spark plugs are new.

Haven't looked at the catalytic converter

surging may be intake or vacuum leak,, which can cause the computer to pulse the injectors on longer than it should. then 02 sensor sees it and says whoa, back off the fuel ... then it goes into a looping surge.. the surging is the computer trying to adapt to a errant condition. also see what thermostat is in it.,. 180 to 195 is good. too cold and it stays in open loop too long.
also u have 2 different temp sensors, one is for the gauge, the other for the computer. test the one on the top front of the engine with an ohmmeter/.
clean the ground wires on top of the thermostat housing, they are important.
coolant temp sensors are critical, and not expensive if in any doubt, replace it.

how long does it take to go into closed loop from a cold start, and can u post the scan data,. ALL OF IT. A good picture of one of the spark plugs might help too.
what are the long and short term fuel trims, MAP reading, knock counts, ect.
also are u getting any smoke out of the exhaust, if so what color. is it using any oil?

Bert_ said:

Temp sensor, map, and tps all appear functional, checked with scan tool. BLM is down around 100.

Click to expand...

Schurkey said:

What is O2 sensor voltage, and O2 crosscounts?


A very-lean Block Learn indicates either a faulty O2 sensor, or excess fuel getting into the intake manifold (which is somewhat unlikely if the coolant sensor and MAP are working properly)

Click to expand...

Bert_ said:

O2 sensor does switch between high/low. I'll have to check what the values are.

Click to expand...

Verify crosscounts. High/low is good, but not good enough. O2 sensors get lazy, slow-to-respond. Crosscounts better be 5--6--7 or higher at idle, more than that as the RPM/load goes up. More is better.

They may also provide false-lean or false-rich indications as they age. O2 sensors need to be replaced periodically, and most folks let them go too long.

Bert_ said:

It does go into closed loop.

Click to expand...

Good.

Bert_ said:

I checked that timing was at 0* with the connection on the firewall unplugged. It advances with the wire plugged in. I didn't check how many degrees. Cap, rotor and wired are maybe a couple years old. Spark plugs are new.

Click to expand...

Good.

Bert_ said:

Haven't looked at the catalytic converter

Click to expand...

You'll want to check that with an infra-red thermometer. The outlet needs to be hotter than the inlet, or the converter isn't converting.

texas tough said:

also u have 2 different temp sensors, one is for the gauge, the other for the computer. test the one on the top front of the engine with an ohmmeter/.
clean the ground wires on top of the thermostat housing, they are important.
coolant temp sensors are critical, and not expensive if in any doubt, replace it.

Click to expand...

He's already verified the temp sensor and the temp sensor wire harness with the scan tool. Using an ohmmeter when a scan tool is available is crazy. Takes longer, doesn't check the wire harness.

texas tough said:

what are the long and short term fuel trims, MAP reading, knock counts, ect.

Click to expand...

He says the Block Learn is all the way down to 100. The computer is in serious lean-command.

Since he says the fuel pressure is reasonable, the coolant temp sensor is working, he's tested the MAP and TPS sensors, and there's no leakage from the injectors--the most-likely cause is a failed O2 sensor. A failed ECM is another possibility.

But I've been wrong before.

I want to know thew long and short term fuel trims,., 128 short temp is perfect. give me all the data and I will tell u what the problem is.
also you can unplug the 02 sensor and get an ohm reading cold, then hot,. then report the numbers if your tool doesnt give them.

texas tough said:

I want to know thew long and short term fuel trims

Click to expand...

On TBIs it's INT (short term) and BLM (long term)

Is it staying in closed loop? And did you verify if when it was surging @ idle it was still in closed loop? I had problem with a lazy O2 once where it was varying but not always fast enough and would sometimes fail criteria and drop out of closed loop at idle. It would stay closed loop driving. I only realized it when I was failing emissions at idle due to rich condition and had to troubleshoot. @idle it was surging (open loop as it turned out) due to marginal fuel pressure (11psi @ no throttle but would dip if throttle nailed) surge was due to idle enrichment kicking in as rpm dropped which, of course, made it run rich for emissions As it was dropping into open loop at idle. That is, at idle, I was running rich due to low fuel pressure due to dropping into open loop and ECU saving idle with dumping fuel.

This is brief summary of longer saga. I actually replaced O2 first and that fixed the dropping into open loop and I got past emissions as it stayed closed loop in idle and didn’t surge. It still would surge at cold start and sometimes stumble. That was fixed with new fuel pump delivering 13psi sold.

remember this , scan tools give you direction to find the fault. unless you like guessing and buying parts, get to the root by testing and not guessing.. sometimes you create additional problems by guessing, because therea ALOT of junk aftermarket parts out there, the more parts u throw at it, the less money u will want to spend on more parts, so you end up with more crappy part.s