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Help me tune up my TBI 350!

76k5blazerr

76k5blazerr

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I want to do a full tune up on my 89 Jimmy with a TBI 350, 223,000 miles, for a few reasons. Ive only had the truck about a month and a half and it is doing well, but usually right when I buy a truck I will fully service it just to know that everything is fresh, have waited a little longer on this one, except I did change the oil when I got it. Anyway, currently the only "problems" I am experiencing with this truck are-
Getting a consistent 9-10 MPG (Normal? Low?)
Truck takes ages to warm up in the cold weather we are getting now - Thermostat
Flashing check engine light, flashes on and off intermittently - PO says its a bad o2 sensor, doesn't seem to effect the way the truck runs.

Anyway, this weekend I think is going to be tune up time. Planning to replace the Tstat to hopefully make it warm up normally again and was planning on doing air cleaner and plugs, maybe wires.

So my questions for the community are, going back to the gas mileage thing mentioned above, the best tank I have put through this thing was like 11 MPG. Now I am used to getting sh!t MPG and it doesn't much bother me, but I was under the impression from reading some stuff here that the TBI trucks got a solid 2-3 MPG better than a carb. Ive read people saying they get like 13-14 riding around town. Anyway the only question I have is this, If 8-10 MPG is NOT normal for a TBI 350 with 223k on it, then I would love to know what to do to fix it.

Also, this is my first TBI truck. Are their any parts on these rigs that need to be replaced as part of a full tune up that a carbed truck doesnt have?

Any thing else I should check/replace as part of the tune up to make the truck run at full potential, it runs fine right now, but I guess every engine has potential to run "better".

The check engine light flashing on and off doesnt really bother me and doesnt seem to have any effect on the truck, but if yall know how to fix that, I would do it. Like i said, PO says its a bad o2, but I feel like a bad o2 would make it runs bad... so I dont know. I dont have a code scanner.

Thanks everyone.
 
I know having a lazy or stuck open thermostat that will delay warm up to normal operating temperature will make it suck gas down a lot faster--the engine controls stay in "cold rich mode" longer and it may seem to run well,most engines will perform good with a richer mixture,but your gas mileage will suffer..

I'm not well versed in EFI issues so I'll leave the other symptoms to those here who are..
 
My engine builder was insistent that I replace the intake manifold gaskets at 100k miles.
 
How much oil are you burning? My 94' tbi Sub was burning a quart per fill up, when it got to 150k miles, Granted, it's a 42 gal tank, so call it every 400 miles.
The engine was rebuilt as a 383 at 150k and I have put almost another 90k.
 
retroblazer said:
How much oil are you burning? My 94' tbi Sub was burning a quart per fill up, when it got to 150k miles, Granted, it's a 42 gal tank, so call it every 400 miles.
The engine was rebuilt as a 383 at 150k and I have put almost another 90k.
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Not burning a large amount of oil. I've driven probably 800 miles since the oil change and there's no drop on the stick. The truck doesn't smoke at all except for a puff on startup after sitting for a while.
 
The puff on startup is likely valve seals-common on higher mileage engines.

9-10 mpg is on the low side, but not outlandish for a truck built like a brick with a high mileage engine, especially if it's not warming up and getting to the most efficient part of its operation. If you have an 02 sensor issue, that can be reducing it a bit. It also depends A LOT on how your truck is set up. Gears, bigger tires, lifts, heavier axles, carrying around a kitchen sink, etc. can have an adverse effect on mileage.

Don't go fancy with spark plugs, Delcos are just fine. Some of the exotic ones can actually cause problems.

Inspect vacuum lines and hoses carefully. While a carb'd engine will do everything it can to not run, an FI engine will do whatever it can to stay running, and that can mask issues like vacuum leaks, poorly adjusted timing, etc.

Do a full inspection of the wires. If they're old go with your idea of replacing them. Wires can acquire cracks and become brittle with age and exposure to heat, sometimes creating performance problems. I've spent time chasing down more than one drivability problem that was just a bad wire.

You've got sensors and parts on the throttle body that sometimes require replacement with age, but if everything works fine just keep an eye on it unless you want to catch a case of "might as well" and do a TBI rebuild.

Always check timing. On any car with adjustable timing, you have people that will play with it, so it's good to get a baseline and go from there. A lot of TBI trucks just have you set it at zero and the computer adjusts it as necessary.
 
12-13 mpg is what I get with my tbi 350 in my blazer
 
Straight highway i'd expect no higher than 16mpg on a stock rig. any amount of city will very quickly destroy that number.

any higher numbers than 16-18 i dont believe, and wont until i drive said vehicle myself.
 
My K5 got down to 8-9 mpg (approximately 230k on it then), did a normal tune-up (plugs, wires, cap/rotor and O2 sensor), mileage came back up to 12ish.
 
mtnman said:
My K5 got down to 8-9 mpg (approximately 230k on it then), did a normal tune-up (plugs, wires, cap/rotor and O2 sensor), mileage came back up to 12ish.
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There is just 1 o2 sensor on these motors right? Is it in the manifold or the Y-pipe? Are they notoriously hard to get out with out breaking something (thinking like manifold bolts)? I'd like to change mine if there is no chance it just going to break something else.
 
Just one o2 sensor,it is in the left side exhaust manifold ,you may need to buy a special socket,but it is no big deal to change
 
Nope.... check out TBI chips also for good TBI info... if doing the O2 sensor i'd upgrade to their unit.
 
76k5blazerr said:
So I'm not likely to crack the manifold or something trying to remove the old one?
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If you really want to, AFS-74 is the Delco part number to run a heated O2 sensor. If you don't know the O2 sensor is the problem, you are worse off replacing it with an unknown quantity (new) vs. keeping what is in there that is working. Heated O2 will net you nothing for power and economy, so the gain is zero. It may go closed loop sooner (I don't know what TBI uses for a closed loop O2 timer scheme) but on a stock tune, stock engine, closed vs. open loop is pretty much meaningless where heated vs. unheated O2's are concerned.

You need to pull the code(s), then diagnose based on that information. Codes don't tell you what is actually wrong, just which sensors the ECM "sees" as reporting improper information. You certainly do NOT listen to PO's, who are notorious for not knowing anything, and doing everything wrong.
 
dyeager535 said:
If you really want to, AFS-74 is the Delco part number to run a heated O2 sensor. If you don't know the O2 sensor is the problem, you are worse off replacing it with an unknown quantity (new) vs. keeping what is in there that is working. Heated O2 will net you nothing for power and economy, so the gain is zero. It may go closed loop sooner (I don't know what TBI uses for a closed loop O2 timer scheme) but on a stock tune, stock engine, closed vs. open loop is pretty much meaningless where heated vs. unheated O2's are concerned.

You need to pull the code(s), then diagnose based on that information. Codes don't tell you what is actually wrong, just which sensors the ECM "sees" as reporting improper information. You certainly do NOT listen to PO's, who are notorious for not knowing anything, and doing everything wrong.
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Im not sure if the check engine light will stay on long enough for me to pull the code. It never stays on more than 30 seconds or so.
 
76k5blazerr said:
Im not sure if the check engine light will stay on long enough for me to pull the code. It never stays on more than 30 seconds or so.
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It doesn't necessarily have to be on to be stored IIRC. You can use a paperclip to jumper terminals A and B on the ALDL connector to get the codes, if you don't know which those are, just google OBD1 GM ALDL connector pinout and I'm sure you will find it.

Note that getting codes is nothing like using winALDL or Tunerpro RT with an ALDL cable to get actual data. Your codes are going to tell you things like "MAP Low", "Rich O2", etc., while the cable will allow you to actually SEE the MAP values being reported, and what the O2 sensor is reading. Among other things, it will also allow you to see the TPS in operation (volts as the throttle position changes) and the temperature being reported by the CTS.

Codes aren't a bad way to get started, the problem starts when people use the codes to start throwing parts at the problem. If you haven't checked, replacing all the potential sensors and senders that could cause problems can get very expensive. And with the current crop of parts being sold for our trucks, oftentimes IMO, a used GM part that you know works, is worth more than an aftermarket part fresh out of the box.
 
Like said get some codes from it before you do anything else. The check engine light only being on for 10 seconds reminds me of the one in my burb.....it is actually "on" all the time, but the bulb only randomly makes connection and lights up.

I really don't think the heated o2 is needed if it is still in the manifold. With a healthy o2 they go/stay closed loop fairly easily.
 
black dawg said:
Like said get some codes from it before you do anything else. The check engine light only being on for 10 seconds reminds me of the one in my burb.....it is actually "on" all the time, but the bulb only randomly makes connection and lights up.

I really don't think the heated o2 is needed if it is still in the manifold. With a healthy o2 they go/stay closed loop fairly easily.
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Interesting, hadn't thought about it being the bulb. So how do I pull the codes? Do I need a code scanner? What was someone saying about the paper clip method?
 
yes you can use a paper clip. It is called flash coding. You will count # of blinks in the series and then look it up the code.

Like 1 flash then pause then 4 flashes is code 14. Goggle flashing coding OBD1
 
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