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TBI driveability issue

80 Chevy K20 4x4

80 Chevy K20 4x4

1/2 ton status
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Jun 4, 2003
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Location
Athena, OR
I just got an 84 K5 Silverado Blazer 4x4 with a TBI 350 transplanted into it, with a turbo 350 trans, NP 203 transfer case, 6" suspension lift, and new 35" Pro Comp XTerrain tires on 3/4 ton axles.

The Blazer will start up just fine and run, but it seems to lack power when compared to my 89 K5 and 87 K20 4x4 rigs. It just seems like it is holding back and not producing the torque. It will accelerate but just not like all the other Chevy 4x4s I've owned in the past. It doesn't burn or leak oil or water, and it doesn't feel like a normal engine miss either.

When the rig is cold and I start it, it doesn't want to idle hardly without having your foot on the throttle a little bit. But after it is warmed up it idles higher then it should when you put the trans in park or neutral. I was thinking maybe the temp sensor might be causing this?

All the wiring seems to plumbed correctly by whomever performed the swap.

There are no codes in the computer.

The exhaust is a dual pipe setup with no cat converter, which I removed from my 89 K5 and 87 K20 and both of them ran fine without it.

It has aftermarket MSD 8 mm plug wires on it. I'm going to check the distributor cap, rotor, plugs, and ignition timing first. Run a vacuum check and look for leaks. Then I'll run a compression check on the motor.

Then next I thought I'd check the throttle body itself, IAC, MAP sensor, EGR, egr solenoid, temp sensor, fuel filter, fuel pump pressure, fuel injectors, etc. It does have a brand new TPS on it.

Is there a common part within the TBI system that might cause it to not produce the normal torque of a TBI motor?

It's been a few years since I had a TBI 4x4, so things are a little bit fuzzy on how to go about checking the TBI system.

Thanks,

Doug
 
Did it ever run strong, or has it always been this way? If it used to be better, it'll be easier to find something that changed. If it's always been like this, it could be something in the tune, or any number of other problems.
 
I just got the K5 this last Sunday, so it has been this way since before I got it.

This afternoon I checked the ignition timing with the EST connector disconnected and I set it to 0*. It was like 6* ATDC. It helped just a very tiny bit, but it still runs like a slug.

It will accelerate and speed up, it just takes it longer to do it.

There are no backfires, stumbling, or hesitation in the throttle at any rpm whether it is on acceleration or deceleration.

Tomorrow I'm going to get a new cap, rotor, and fuel filter just for the heck of it.

Then I'll run a compression check on all 8 holes and see if something is amiss there.
 
Fuel filter and cap and rotor are notoriuosly overlooked as needing maintenece so that's a good start.

Then check fuel pressure.

Then when you find nothing wrong you will notice when they did the conversion they did not put in a VSS or PK/Neutral switch. No one does... the VSS will make it idle better, go into gear better and run better.
 
I can't remember without looking behind the speedometer, but is the VSS attached to the speedometer or can I get the VSS by itself and install it?

Also, can you refresh my memory on how the VSS interacts with the ECM to help the vehicle run better? I know it senses speed, but what does it actually do for making the motor run smoother?

Thanks for the help,

Doug
 
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1984 Chevy trucks do not have a VSS behind the speedo. 1987 up does.

Conversions are usually easier to put one in speedometer cable, has male and female and just bolts in, then run wire to ECM.

Vehicle Speed Sensor gives feedback logic to ECM telling it if it is 0 mph or close to invoke idle logic and also tells ECM when at speeds the load on engine from MAP. I have done conversions and run fine... then I added a VSS for kicks and could not beleive how much better it behaved. Driving on street, highway was improved, but idle stability in my off roader was incredible. Off roading, slow, crawling type bumping over rocks stuff it would stall once in awhile or hit stall saver and flare up RPMs. VSS made it much more stable!
 
Okay, this weekend I tore the dash cluster out of my K5 and it had a VSS hooked up behind the speedo and plugged into the wiring harness for the TBI system that a previous owner had transplanted into the rig.

I changed it out with a working one from a wrecking yard though, just on the off chance that the one that was in there was defective.

I had to change out the steering column due to a bad upper bearing, and it had the Park?Neutral switch hooked up to the wiring harness also. I moved it over to the functional steering column when I installed it.

So what do you think my next move should be?

Thanks for the help,

Doug
 
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I'd clear codes by disconnesting the ECM or batterey for a minute, start er up and drive it! How'd it do? Is it a stock chip in ECM? Should be improved! :waytogo:
 
I haven't looked at the ECM yet, or checked for codes even though I don't have a SES light coming on except for at startup.

I did however do a compression check on the motor cold, and this is what I got - which I think it a little on the low side;

Cylinders

1. 115#
3. 115#
5. 120#
7. 125#

2. 125#
4. 125#
6. 120#
8. 120#

If I decide to drop the 91 motor that has an Edelbrock Cam, intake, and carb from a 86 K5 I'm picking up tomorrow into this K5, would I have to do anything besides changing the distributor and disconnecting the ECM to run the carb'd motor until I can swap the TBI intake and get a custom chip burnt since the 91 motor has a performance cam in it?

The 84 K5 I have has a better body on it and looks better then the 86 K5, so I was thinking of transplanting the 91 motor, 700r4, and NP 208 from the 86 into my 84.

Thanks for the help,

Doug
 
Just got back from Bonners Ferry with an 86 K5 Blazer that I traded my 92 Pontiac Firebird Trans Am for.

I did even touch the throttle plates of the TBI when doing the compression check, it was all just cranking the motor.

The motor was cold and hadn't been started for a few days when I checked it.

Was I supposed to check the compression by holding the pedal to the floor? It's been years since I had to mess with compression checks on rigs, so please excuse me if I did it wrong....lol

Actually tonight when I drove it up on the mountain to my family ranch where I put the 86 K5, I could tell the motor isn't firing correctly, so in the morning I'm pulling the cap and rotor, plug wires, and putting new ones on. It already has brand new AC Delco spark plugs in it and they look great. I'm thinking since the contact on the distributor cap for the coil wire is corroded, I bet the inside of the cap and the rotor are dirty also.

Thanks,

Doug
 
Open throttle blades and a warm engine will raise those numbers.

Bonners ferry is about an hour from me!

If you have never changed cap and rotor it very well may be shot!
 
Since I just got the Blazer a couple weeks ago, I finally put new plug wires, distributor cap, and rotor on the motor. The old ones were really dirty and now it runs smoother, but it still does not perform like my other TBI chevys did in the past.

I'm sure the IAC is dirty or worn out, because when the rig is cold and you start it, it won't idle on it's own unless you run it with your foot on the throttle a little bit for a few minutes. Then if the rig is warmed up and you start it, it will idle at a normal rpm, but once you put it back into park or neutral after it's been drove, the idle rpm will be at a higher rpm for some reason. And that's only when you put it in park or neutral after it's been driven.

I do know that the wiring harness has the plug for the 700R4 trans which is not in the Blazer at the moment, it has the TH350 in it. I'm taking the 700r4 and NP208 t case out of my 86 K5 and transplanting it into this 84. Will that have anything to do with the idle quality or driveability issue, or is the transmission plugin only for the torque converter lockup solenoid?
 
Silverado_K5 said:
Since I just got the Blazer a couple weeks ago, I finally put new plug wires, distributor cap, and rotor on the motor. The old ones were really dirty and now it runs smoother, but it still does not perform like my other TBI chevys did in the past.

I'm sure the IAC is dirty or worn out, because when the rig is cold and you start it, it won't idle on it's own unless you run it with your foot on the throttle a little bit for a few minutes. Then if the rig is warmed up and you start it, it will idle at a normal rpm, but once you put it back into park or neutral after it's been drove, the idle rpm will be at a higher rpm for some reason. And that's only when you put it in park or neutral after it's been driven.

I do know that the wiring harness has the plug for the 700R4 trans which is not in the Blazer at the moment, it has the TH350 in it. I'm taking the 700r4 and NP208 t case out of my 86 K5 and transplanting it into this 84. Will that have anything to do with the idle quality or driveability issue, or is the transmission plugin only for the torque converter lockup solenoid?
Click to expand...
Is the timing set to 0 with wire disconnected?

You very well may have a dirty IAC or bad IAC. Hard to tell bad unless your looking at it or data. Have you done the TPS and min air setting?

If the wires are not connected to 700r4 for TCC it should set a code. Since the newer th400 has an electronic kickdown the 700r4 may too... I don't know for sure... Dave W ?
 
I set the timing to 0* with the EST connector disconnected.

The TPS is new from the PO before me, but I need to verify if it is adjusted properly or not?

I need to look and see if someone before me has knocked the plug out in front of the minimum air screw. I haven't touched it yet.

I bought a brand new IAC yesterday, but will put in the the 84 K5 this evening after work. I was busy yesterday getting the 700r4/208 ready to come out of the 86 K5 so I can swap it into my 84.
 
You should at least check what voltage it is at idle and min air adjsutment will effect idle. When TBI wear the throttle plates close... there needs to be min air to let gas through especailly on TBI because injector is on top of throttle blades.

All EFI cars have a minimum air setting, this setting usually always changes TPS voltage. The 2 have to coraspond.

http://www.thirdgen.org/tpimod2
 
If the TPS is out of adjustment from where it's supposed to be set at, will that cause the motor to run sluggish at any rpm or speed?

I still haven't had a chance to look at the TPS, or replace the IAC with the new one I bought, but thought I better ask about the possibility of the TPS causing it to run like crap from idle all the way up to the top end.

Thanks,

Doug
 
+1 on the fuel filter and pressure...:waytogo:
 
Silverado_K5 said:
If the TPS is out of adjustment from where it's supposed to be set at, will that cause the motor to run sluggish at any rpm or speed?

I still haven't had a chance to look at the TPS, or replace the IAC with the new one I bought, but thought I better ask about the possibility of the TPS causing it to run like crap from idle all the way up to the top end.

Thanks,

Doug
Click to expand...
If it is bad yes! Out of adjustment,no. It should be .54 volts at idle but can be up to 1.0 volt at idle.

Have you checked fuel pressure?

It may be time to buy a scan tool to look at data or buy a cable to record data on a laptop... or stop by my house...
 
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