Who put V-Tech on my L31?

[redfishsc]

So I have a problem going on that I'm only just now caring enough to start investing. Lately (the past 6 months honestly) my low-rpm power has decreased noticeably but not such that it effects driveability.


1999 Suburban, 5.7, engine is stock except for upgraded MPFI injectors. 2 years ago I put on reman heads and upgraded injectors, new lifters.

Problem: Reduced power at lower RPM, especially noticeable if you try to do WOT.
If I'm doing WOT I can tell the truck isn't developing it's normal power until suddenly around 4,000rpm "V-Tech kicks in, yo". I mean it's nearly identical to how the V-Tech feels in my Honda 3.5l V6. About 4K rpm you get this sudden "awakening" of power. Whatever the problem is, it isn't a problem at 4k+ RPM.

My gut feeling is that this is a fuel/air ratio thing because it feels a lot like a bad vacuum leak that wreaks havoc on the ECM's algorithms, except that the truck does not stumble or misfire.

I am showing some Cat bank sensor codes (can't remember which) so it is possible this is cat related.

It doesn't stumble at low rpm, doesn't stumble at any time driving, no misfires detected in OBD history. No obvious vacuum leaks that brake cleaner or WD40 can find.

Trans is original (she old... 234k) but lines are boosted as much as my tuner will allow, but I did that several years ago.


Fuel filter is a few years old and will be replaced soon but I'm not sure that it's the cause here anyway.

 

[stutaeng]

Cat bank sensor code huh? I would start there.

If catalytic converter is not working properly the 02 sensors are reporting this to the ECM and it is likely cutting fuel. Monitoring fuel trims while taking it for a spin would be my first suggestion.

Do you have access to a scanner that has live data? Maybe a friend of a friend has one or knows one who does?

 

[redfishsc]

Gents, just to update this. I'm not entirely sure if I'm fixed but I did find some definite smoking guns.

MAP sensor was utterly schmutzed up. I cleaned it with MAF cleaner and that made a significant improvement but didn't eliminate the issue entirely.

Throttle Position Sensor threw a "low voltage" code yesterday for the first time. I removed, found no problem with the connection, but cleaned with electric spray cleaner. Erased the code, put 40 miles on it, code didn't return. New sensor coming anyway, so easy/cheap that it's worth doing for preventative anyway.

Lastly, the O2 sensor.

Bank 1 sensor 1 was definitely shot. Was throwing a "no communication" code, which I erased and replaced the sensor it just now. The sensor's "fins" were fairly plugged.

Code didn't return after a 10 minute idle and some throttle-play. I'll drive it after lunch to see.


Hopefully one of these is the fix and not some f&#king mouse chewing up a wire somewhere I can't see.

 

[thegawd]

I was just about to suggest the tps.... if its faulty and sending incorrect info our trucks aren't smart enough to figure it out. No codes are provided as it's running correctly with the wrong info.... it's a mechanical problem inside an electrical device. Maybe it was the wiring but I dont buy it. In my experience it's almost never the wiring in these old trucks unless of course a mouse.....

you can test the tps a few different ways. On my cheap obd2 reader I can graph the TPS. It should rise in a smooth uninterrupted line as you open the throttle. Any jumps, plateaus or missing data and you have a faulty TPS.

When I put a new tps on my 97 burb I accidentally did a burn out.

I hope it's just that simple.

Take care

Al

 

[redfishsc]

Ok gents, it appears that I am fixed.

I had three definite issues all contributing.

Dirty MAP sensor
Bad 1,1 Oxygen sensor.
Bad (or dirty
) Throttle Position Sensor.


I cleaned the MAP and swapped the other two for new Delco and it's running like a champion again.

 

[stutaeng]

Glad you figured it out.

On the O2 sensors, they recommend you replace them in pairs. (both both upstream and downstream bank 1, for example.) Rockauto has the best prices...not sure how much that matters?

 

[Supercharged111]

Glad you figured it out.

On the O2 sensors, they recommend you replace them in pairs. (both both upstream and downstream bank 1, for example.) Rockauto has the best prices...not sure how much that matters?

They also say do ball joints in pairs, etc. I smoked an O2 sensor last year on the dually. O Reilly's only had 1 in stock in Hays, KS. Pulled both, one was brown one was black. Replaced the bad one and all is well. Never threw an O2 code then, nor did the one on my blue truck that went full lean. Again, I just did the one, but it makes me suspect a problem on one bank if it's killing O2 sensors.

OP the reason your truck woke up right at 4000 is because that's the point where the truck goes full MAF and ignores the MAP sensor.

 

[redfishsc]

I put 600 miles on her this weekend for a family funeral and she ran like a champion the whole time, idled slick as glass and has great throttle response


Now, however, the brake light is coming on occasionally when I hit the brakes and I seem to feel something slightly "not quite right" on the pedal. I'll look around and start a new post if I can't find the issue. Brake fluid still full.

 

[1ton-o-fun]

Sorry for your loss.
Glad the truck is running better. -Other than the brake issue, lol.

 

[Schurkey]

No obvious vacuum leaks that brake cleaner or WD40 can find.

DO NOT use "brake cleaner" to test for vacuum leaks!
1. Not all "brake cleaners" are flammable. If the spray won't burn when sucked into a vacuum leak, you don't get any indication that something's wrong--the engine still runs lean.
2. Some brake cleaners break down into highly poisonous phosgene gas when heated. Folks get seriously fooked-up from phosgene. Germans used it in WW I as a nerve gas.

WD40 is nearly as useless, but at least it won't poison you like some brake cleaners.

I use aerosol carb cleaner most of the time, occasionally propane. Either one burns, so when "inhaled" by a vacuum leak, the extra fuel offsets the extra air, and the engine speed increases. (OR, the engine is already getting extra fuel to compensate, so the additional carb spray or propane causes an over-rich mixture, decrease in rpm.) Either way--Positive proof of a vacuum leak.