hey guys, i have a 1988 k5 blazer tbi (i know not a third gen but you guys seem to be tbi wizards so i came here) recently it has been running super rich and sluggish, no smoke but i went from getting 15 mog avg to 5mpg and the exhaust has a strong gas smell, i have replaced everything. o2 sensor, cts and connector, tps connector and verified proper voltage, map connector, iac and connector, rebuilt tbi, new fuel pump reg and filter, new air filter, new pcv valve, new egr, you name it, i am completely stumped and need help, along with it running rich it has been idling high and jumping 700-800 then drops to jumping 600-700 rpm. please help!

 

You need to go to a site like gm-trucks.com, or one exclusively for Blazers. This site is for 3rd Gen F Bodies, and yes, it makes a difference.

Furthermore, just putting "Need Help Fast" in the title helps nobody, and actually discourages readers.

And have you checked for codes, or are you just changing parts???

 

sorry i just put need help fast because 5mpg is gonna burn a huge hole in my wallet, yep checked zero codes and checked to confirm the sensors were bad before i replaced them and checked to make sure new ones are good with voltmeter, sorry im just looking for help anywhere because im stumped, it was fine for a while now its all screwed up.

 

Connect a scan tool, view the data stream--verify EVERY sensor and computer output. Check fuel trims. "Codes" can be helpful. The REAL diagnostic power is in the data stream.

How old are the usual "tune-up" parts and procedures? Distributor cap, rotor, plug wires, spark plugs, PCV valve. Verify initial timing and electronic advance. Check the charcoal canister and fuel tank venting system. Assure the EGR is working properly.

Is fuel spraying out of the injectors...or gushing?

You don't have a bigass stinky puddle under the vehicle every time you stop?

Gasoline thieves in your neighborhood?

Brakes really hot and stinky?

5mpg can't be that hard to track down.

 

Dist cap, rotor, plugs, wires, pcv valve all brand new, all venting seems proper, egr is new and I disconnected the vacuum line when it was idling poorly, which didn’t change the idle. I replaced the connectors for both intake manifold grounds by thermostat and that seems like it may have fixed the high rough idle, but it still runs a bit sluggish warmed up, I do not have a scan tool unfortunately, but I’m going to try to test from o2 connector with engine running, any idea what volts it should be?? I also heard that Bosch (what I have) o2 sensors do not usually work well with TBI’s, does anyone know anything about that or is it just made up.

 

"New" doesn't mean the system works properly. If the passages cast into the head or intake manifold are plugged with carbon, the EGR quits working, detonation increases, and the computer retards the timing.

You need access to one. Trying to fix a computer controlled vehicle without being able to communicate with the computer is two steps away from hopeless.

Between .1 volt and .9 volts, changing back and forth as rapidly as possible. That assumes you're using a voltmeter with AT LEAST 10K ohms input impedance, which is fairly common now. Didn't used to be. Often about 6--8 changes through .45 volts at idle, maybe 20+ at higher rpm.

 

I would check the injectors. If they're original, they're ancient. One or both could be leaking.

 

You can test for leaking injectors w/o replacing them. Take the air filter off, turn the key on to run the fuel pump, observe the injectors. Are they leaking?

Answered, for $0.00. TBI injectors are typically,incredibly reliable.

 

This site has always been one of the best go to sites for TBI, TBI modifications, and Prom Tuning.

GM TBI is GM TBI, does not matter much if it is a F-car, a Crossfire, a light truck, a RV, medium duty truck or a boat, the basics are all very close to the same.

 

Checking 02 response and closed loop function on OBD1 GM like TBi is very simple. Jump A to B in the ALDL connector, start the engine. During open loop warmup the SES light will flash once a second. After it warms up and goes into closed loop the SES light will the track with the 02 sensor.

Beyond that simple fuctional test will need a scan tool capable of monitoring the ECM.

New does not mean good when it comes to sensors and components. Furthermore you are possibly dealing with a bad harness connection. You said you rebuilt the TBI. Did you pull it off the manifold? The base gaskets often degrade and create a vacuum leak, right into the MAP port, resulting in over fueling. If the return line to the tank becomes restricted the engine will overfuel. Have you verified the base timing with the EST wire disconnected? It is possible the distributor loosened and turned, retarding the ignition timing. Have you verified the catalytic converter is not plugged? I have a 0-10 psi gauge with an adapter that threads into the 02 sensor bung to measure backpressure. Also if the truck has air injection, a mulfunctioning diverter valve that is pumping air into the manifolds at all times will make the 02 sensor read lean even with the engine running very rich. What do the spark plugs look like? If the engine is burning enough fuel to get 5 mpg, I feel like they would be choked with black carbon and fuel fouled. I have seen the EST circuit in a module fail without codes, resulting in limp home timing mode. I have seen new modules DOA in that regard as well.

 

I've been meaning to ask, where can I get a scantool for 80s GM OBD1? And which one do you recommend to buy?

 

A cable for tunerpro rt, snap on mt2500 or otc pathfinder.

 

I had a Snap-On MTG2500 for almost 20 years, purchased used from a seller on eBay, with additional adapters and software cartridges also purchased used from eBay.

When the MTG2500 died, I replaced it with a Snap-On Solus Pro, purchased--you guessed it--used, from a seller on eBay. This was a couple of years ago, I spent $350 delivered to my door. But Senile Joe has "fixed" that, an equivalent tool is about a hundred dollars more now than it was then. 'Course there's always local pawn shops, Craigslist, and the occasional "deal" on eBay.

I gotta say, the Solus Pro is far more user-friendly than the old '2500 units. More memory, bigger screen, astonishingly better graphing (the MT2500 has NO graphing, the MTG2500 has rudimentary graphing.) Of course, just like the '2500, and the original Solus, the Solus Pro also has NO support from Snap-On. NO updates, NO repairs, NO parts except what's also used on the Solus Legend or whatever the current version is.

My Solus Pro has 8.2 software, good for 1980 1/2 to 2007. Engine, ABS, instrument clusters, traction control, air bags...everything with output to a scan tool, which is vehicle dependent. Older vehicles had less computer-controlled stuff in them. My one disappointment is that it will not auto-bleed the ABS of my '97 K2500. That feature wasn't added until after the 8.2 software. I can verify ABS sensors and read ABS codes, but not auto-bleed.

 

"All fuel control is based of the MAP Manifold Absolute Pressure sensor and rpm readings."

As found on this TBI trouble shooting page:
https://harristuning.com/Tbi/troubleshooting/

 

Old post but it is also dependent on the TPS, coolant temp sensor and 02.