Code 44 Lean Exhaust '88 K1500 5.7 TBI

'88 K1500 regular cab/long bed, 700 R4, 3.42 gears, 5.7 TBI

I get a code 44 Lean Exhaust at highway speed--55 mph +.

Code never sets at lower speed. Sets as predictable as the sunrise when faster than 50--55 mph.

Scan tool shows block learn and integrator numbers trying to richen mixture at highway speed, but pretty normal below that. Shows zero 02 crosscounts at highway speed, and low voltage--but lots of crosscounts and normal, varying voltage at lower speed. O2 sensor therefore "seems" good.

Disconnected AIR hose on driver's side manifold. No change--still lean at highway speed.

Fuel pressure measured at the fuel filter OR at the TBI inlet fitting is 11.5 psi from idle to 90+ mph. I'm aware that the 5.7 TBI engine is no hot-rod, but I have all the power I expect from it; and it never misfires or runs rough. It does crank longer than I think it should, but fuel pressure comes right up when ignition is turned on. Might be that I have unrealistic expectations for fast starting. Ignition pattern looks good on an oscilloscope when powerbraked in the shop. Does not "seem" to be lean and lacking power--but if there's more power to be had, I'm all for it. Knock sensor shows "some" activity, but not much, and only when accelerating especially towing the boat.

Have run SeaFoam in the gas tank; did not make a bit of difference. Injectors have never been "professionally" cleaned, at least not in the last 130,000 miles. Vehicle has 300K miles on it. Fuel mileage has NEVER been impressive since I bought the truck at 170K; when I was commuting 60 highway miles per day, I was getting less than 14 mpg (70 mph more-or-less).

Any ideas on why an engine would run good, have reasonable (but certainly not excessive) power, but still set a lean code; and have the O2 sensor showing a lean condition?

Quick update:

I've been doing a lot of work to this truck lately:
A. Pulled dash out, repaired blower box-to-windshield plenum gasket

B. Replaced instrument cluster to fix intermittent speedometer/odometer

C. Replaced rotted muffler and tailpipe

D. Installed different TBI unit; replacement is "used" and from a slightly newer 5.7L truck--the casting is a little different; but other than that it's in unknown condition. IAC and TPS are within spec; and it squirts fuel at idle in the same pattern or cone-shape as the original TBI.

Truck runs just like before:

1. Starts reliably after cranking awhile.

2. Has all the power I'd expect around town. O2 crosscounts of 15+; block learn and integrator numbers about what I'd expect; and reasonably close to 128.

3. Today, at highway speed, flat ground, I can watch the block learn and integrator numbers climb, and climb, and climb as I increase speed from 50-ish to 70 or so. While this seems like an improvement, I have to consider that the outside temperature is different, and I'm not considering headwind/tailwind. No codes set, but clearly the computer isn't happy based on the fuel trim numbers going to "rich command".

4. At 70--72 mph, my O2 crosscounts go to 0; and the engine sets a Code 44 after about twenty seconds. Engine does not stutter, miss, shudder, vibrate, or in any way "act" lean.

I don't know what else to do. When I get back from my next out-of-state trip in about three weeks, I'm popping the fuel pump/hanger out of the tank and see what it all looks like. Most likely replace the pump.

AC fuel pump is about $50 out of Amazon including strainer. There are Carter and Airtex replacement pumps and strainers for about the same money. Some folks say the high-pressure TPI pump is a bolt-in improvement; I'm a little concerned about the added pressure going to the TBI regulator--but--I already own one of those pumps so I'll probably buy a strainer and try it.

It seems like you have eliminated many of the usual suspects, or at least the easier ones. It was a good idea to disable the A.I.R. for the test. I’ve had a couple TBIs with similar issues and pulled my hair searching for causes. Another place to look is the PCV valve. Just for grins, the next time you take it out for a scanner run plug the PCV or clamp off the vacuum line and see where the numbers go. Also, take a look at the MAP numbers the during the next test run. The MAP should respond to throttle opening in a fairly linear manner and not hit a plateau.

I didn’t see in your posts whether the O² crosscounts change once the code sets. Since the ECM should be going to backup fuel mode once the code sets the crosscounts should change (the sensor is still active regardless of a code). If not, the fuel delivery could be maxxed out.

What happens to injector pulse width as speed increases? If the injectors go to much over 10 mS there may be a fuel delivery problem upstream of the injectors, or the injectors may be undersized. Make sure the replacement injectors are at least 61 PPH instead of 55s from a 305.

With no O² crosscounts it sounds like the system is going lean, or at least the sensor thinks so. Has the sensor been replaced lately? Your one-wire sensor may be getting a little weak. A typical zirconia sensor will slowly lose signal generation as it ages, telling the ECM that it's going lean. Even at that, the fact that the BLM/INTs near 128 in lighter load driving one might think everything was normal since the sensor can at least generate good signal under those conditions.

I know the TBIs are not easy to connect a pressure gauge, but it may be worthwhile in this case. If the fuel pump can't maintain adequate pressure (11-13 PSIG) at higher flows, it might be causing a lean condition. I would be remiss if I didn’t ask the obvious, but has the fuel filter been changed lately? One way to check your pump performance is a flow test. If the pump cannot move at least one pint of fuel in 15 seconds at rated pressure, it may have a problem. That problem could be a filter, pickup sock filter, or low voltage at the pump. Of course, it would be good to inspect the fuel lines for kinks and flat spots.

i have a similar problem as you with the exception that mine cuts out under a load unless you floor it was told the coolant temp sensor was the culprit so i replaced it helped a very little but there again similar prob but not same if you get it fixed let me know may fix mine to

Another place to look is the PCV valve.
Good catch. I sprayed it out with carb cleaner, but did not actually test operation.
Also, take a look at the MAP numbers the during the next test run. The MAP should respond to throttle opening in a fairly linear manner and not hit a plateau.
Yup. Seems to work just fine including showing appropriate voltage when KOEO.

I didn’t see in your posts whether the O² crosscounts change once the code sets. Since the ECM should be going to backup fuel mode once the code sets the crosscounts should change (the sensor is still active regardless of a code). If not, the fuel delivery could be maxxed out.
Crosscounts go to 0, SES light comes on soon after, crosscounts stay at 0 and O2 voltage is very low. Yup, just as if the thing isn't getting fuel.

Course, I can drive 90 mph that way...

What happens to injector pulse width as speed increases? If the injectors go to much over 10 mS there may be a fuel delivery problem upstream of the injectors, or the injectors may be undersized. Make sure the replacement injectors are at least 61 PPH instead of 55s from a 305.
I can't check pulse width with my scan tool. Both TBIs have the right part number for the 5.7L injectors.

With no O² crosscounts it sounds like the system is going lean, or at least the sensor thinks so. Has the sensor been replaced lately? Your one-wire sensor may be getting a little weak. A typical zirconia sensor will slowly lose signal generation as it ages, telling the ECM that it's going lean. Even at that, the fact that the BLM/INTs near 128 in lighter load driving one might think everything was normal since the sensor can at least generate good signal under those conditions.
The O2 sensor has something like 70K on it; it went in with the crate engine. I'll acknowledge that it could be weak. Even so, it seems to work just fine at lower speed. I don't think the sensor is the issue, but I'm getting more and more convinced that the fuel pump is weak, the sock is plugged, or the little stub hose is split.

I know the TBIs are not easy to connect a pressure gauge, but it may be worthwhile in this case. If the fuel pump can't maintain adequate pressure (11-13 PSIG) at higher flows, it might be causing a lean condition. I would be remiss if I didn’t ask the obvious, but has the fuel filter been changed lately?
Did that last summer--Pressure is steady 11.5 at 70 mph. Current draw is slightly high, but the 'scope pattern (fuel pump current draw) looks good; and the pump RPM is reasonable.

Soon as I get back, I'm yanking the pump out for an inspection.

Here's the difference between the original and the replacement throttle body. I don't think there's a functional difference; I just thought it would be interesting to see the change made between '88 and perhaps '92--'93 or so. Part numbers in lower left corners are the NAPA p/n for the base gaskets.

http://hbassociates.us/5.7_TBI_original_sm.jpg

http://hbassociates.us/5.7_TBI_newer_sm.jpg

Any exhaust leaks up stream of the o2 sensor? This can cause a lean o2 reading.

Any exhaust leaks up stream of the o2 sensor? This can cause a lean o2 reading.
Not that I know of. Been a little suspicious of the AIR check valves, though.

I should look at that again. Thank you!

Have you hooked a Vacuum gauge up yet, to verify steady engine vacuum and no leaks?

I'm assuming you have a 1 wire O2 because of the age of the vehicle, which takes time to heat up. Have you hooked the O2 up to a scope and looked at the switching of the sensor output at idle as well as under load? There should be a fluctuating voltage between 0 and 1 volt, without actually ever reading 0 or 1 volt. Your switching point should be somewhere around .4-.5 volts.

In your remarks about disabling the A.I.R. on the left side (where the sensor lives) I assumed you capped or plugged the lines so that no air would be admitted. This would eliminate the check valves on the system as well.

ADM Mankato? Interesting. I'll be talking to the guys at Marshall this week.

Have you hooked a Vacuum gauge up yet, to verify steady engine vacuum and no leaks?
No. Engine idles dead-smooth; and no ignition breakdown visible on the 'scope while power-braking. But, all things considered...I suppose it's worth looking at. Thanks.

I'm assuming you have a 1 wire O2 because of the age of the vehicle, which takes time to heat up.
Yup. Falls out of closed loop at stoplights, but goes back to closed loop within 5 feet of taking off after the light turns green.

Have you hooked the O2 up to a scope and looked at the switching of the sensor output at idle as well as under load? There should be a fluctuating voltage between 0 and 1 volt, without actually ever reading 0 or 1 volt. Your switching point should be somewhere around .4-.5 volts.
No, haven't looked at the o2 on the scope. Voltage varies on the scan tool, though. I must say that it spends more time "lean" than it does "rich"--but it will show rich on and off at less than highway speed.

In your remarks about disabling the A.I.R. on the left side (where the sensor lives) I assumed you capped or plugged the lines so that no air would be admitted. This would eliminate the check valves on the system as well.
I did plug the check valve with duct tape; this would have been some months ago. No change. I'd forgotten that. This thing has me forgetting what I've done and what I haven't...

ADM Mankato? Interesting. I'll be talking to the guys at Marshall this week.
My uncle worked there (ADM Mankato) for about a thousand years. Now retired.





I wonder how many folks "get" the "Group W Bench" thing in your signature.






I don't want a pickle, just wanna ride on my motor-sickle.

I don't want a pickle, just wanna ride on my motor-sickle.

That's the idea. I'm thinking you need to post some 8 x 10 glossy color photographs with the arrows and the circles so we can get a better idea of what you've done.

FWIF, my brain can't get away from the fuel pressure/flow thing.

Replaced fuel pump and strainer: No change.

Replaced O2 sensor: Was seized in the exhaust manifold. Had to work to get it to un-screw. Once out, I could see the porcelain inside was broken. It MIGHT be that the porcelain broke during dis-assembly; but there were no shock loads--just a huge amount of force from the wrench, applied by hand. I wouldn't have expected that to break the guts of the O2 sensor, but it's a possibility.

At any rate, the SES light no longer comes on at highway speed. Towed the boat 250 miles to the Family Reunion, and 250 back home.

Have not had the scanner connected, though, so I don't know about actual voltages or the block learn/Integrator fuel trim numbers.

Strange that the O2 sensor could react at lower speed/rpm; but give false lean indications at higher speed. I just don't get it.

Replaced fuel pump and strainer: No change.

Replaced O2 sensor: Was seized in the exhaust manifold.


Strange that the O2 sensor could react at lower speed/rpm; but give false lean indications at higher speed. I just don't get it.

So how many miles on this O2 sensor 300,ooomi ?

removal/loosening of the O2 sensors with the exhaust hot and using a cold socket or box wrench works for me..


interesting post with your testing and work ..I would have just replaced the O2...

So how many miles on this O2 sensor 300,ooomi ?

The O2 sensor has something like 70K on it; it went in with the crate engine.
The truck has about 300K, however.

removal/loosening of the O2 sensors with the exhaust hot and using a cold socket or box wrench works for me..
Thanks for the tip.

interesting post with your testing and work ..I would have just replaced the O2...
I'd have flung one in a long time ago if the thing didn't give me good crosscounts at lower speed. Didn't seem to act lazy or "off".

The truck has about 300K, however.


Thanks for the tip.


I'd have flung one in a long time ago if the thing didn't give me good crosscounts at lower speed. Didn't seem to act lazy or "off".


the O2 sensor could have been damaged by install , dropped, wires are very sensitive to damage. any coolant burned by the engine kills these O2 sensors quickly..