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Ongoing TBI problems

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scrappy88

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So I've been dealing with my '88 K5 running crappy for several years now and finally trying to get it fixed up to be a short term daily driver.

It's been running with a low rough idle both in and out of gear, and stumbling just off idle. It does run rich, according to my nose. From what I remember, I have replaced the following over the past few years:

EGR Valve
EGR Solenoid
O2 Sensor
IAC Valve
TPS
Coolant temp sensor (last week)

I've checked for vacuum leaks and can't find any. The truck is stock except for a Turbo City bored TBI body, stock injectors.

Now, when the truck is warmed up and I disconnect the coolant temp sensor, the idle smooths out to about 950rpm and 650rpm in gear. It runs great with the sensor disconnected, and the exhaust doesn't stink. I have replaced the coolant temp sensor twice now and still it runs like crap when it's connected.

What else should I be checking?
 
Just out of curiosity, have you tried a stock TBI throttle body to see if it runs better?

I'm just wondering if the computer needs adjustment for the bored unit. Otherwise, it sounds like you are on the right track... although I would think the computer *should* see that it's running rich. What t'stat are you running? Stock temp? I have heard that if you put in a cooler t'stat, that the computer will never get into closed loop mode, and run rich.

Good luck,
Clay
 
It *should* be the correct temp thermostat.. but I haven't had to touch it in years. Wouldn't hurt to replace it.

I don't have a stock TBI unit to try. I seem to remember the truck ran fine after I swapped the turbo city unit in 6 or 7 years ago.
 
What does the temp gauge read when the truck is warmed up? Should be about 200* +/- 10*.

I had this issue with mine because Jegs sent me a 165* with the TBI topend kit I bought. Once I replaced it with the stock 195* it ran alot better. And make sure you arent putting any teflon on the temp sensor as well. I was told it jacks up the reading. I used a little on the top threads to get a good seal, but that's it.
 
The temp gauge shows around 200-205 degrees when warm. When I replaced the temp sensor last week I didn't use anything to seal it. I was excited because I thought I finally had found the issue, until I plugged in the new temp sensor and it was back to running like crap.
 
You should put it on a scanner and observe all the sensors while they are live.
 
Ok I finally built myself an ALDL cable and hooked it up. I'm new to WinALDL so I don't know what all of the flags mean. I saw a DRP Occured flag, the Rich flag goes on and off all the time, BLM enable flag. What's strange is that the ECM comes out of closed loop mode when stopped at a light, and then goes back to closed loop after accelerating. Is this normal? I logged a short drive but don't have the data with me at the moment.
 
scrappy88 said:
I saw a DRP Occured flag, the Rich flag goes on and off all the time, BLM enable flag. What's strange is that the ECM comes out of closed loop mode when stopped at a light, and then goes back to closed loop after accelerating. Is this normal?
Click to expand...

All that is normal.

Can you check fuel pressure and timing?
 
So it is normal for the ECM to come out of closed loop at idle?

It doesn't have headers. Stock manifolds with a single wire o2 sensor. The timing is set at 0 degrees with the wire disconnected. I don't have a way to test the fuel pressure.

I'm thinking of just rebuilding the TBI, and having the injectors cleaned, put on an adjustable fuel pressure regulator and the pressure gauge adapter while I'm in there.

I have a feeling this is a vacuum leak issue but I haven't been able to find one.

Thanks for all the replies so far!
 
scrappy88 said:
So it is normal for the ECM to come out of closed loop at idle?
Click to expand...

Yep, sometimes. I don't know the details of when and why it does that but it does. Have you verified the CTS has a good ground and power going to it? A broken or bad ground might cause a good sensor to act up.
 
It has a good ground. I didn't use anything to seal the threads. Also the readings coming from it are consistent and accurate; they don't jump around at all.

I'll update this after I get the TBI rebuilt and injectors cleaned.

Thanks!
 
The ECM could drop out of closed loop if the O2 sensor does not stay hot enough. This is more comon w/ headers and a single wire O2 sensor. Is your O2 sensor in the stock loction (close to the manifold)?
 
scrappy88 said:
It has a good ground. I didn't use anything to seal the threads. Also the readings coming from it are consistent and accurate; they don't jump around at all.

I'll update this after I get the TBI rebuilt and injectors cleaned.

Thanks!
Click to expand...

For reference, the cts has an actual ground wire built into the harness so you can use sealant, tape, etc on that one.
 
The o2 sensor is in the stock location. It's a new Bosch sensor and I've heard from several sources now that the Bosch do have quite a few problems and to stick with ACDelco. Would it be worth while to just wire it and swap in a heated sensor?
 
I built an interface cable and use WINALDL also. I have headers on a 1987 Jimmy with a 350 TBI and could not get into the closed loop so I built a harness for a 4 wire O2 sensor then installed it. Now it goes to the closed loop and stays there perfectly.

DSC01336.jpg
 
Just an update to this thread-- I swapped in new plugs, dist cap and rotor and MSD superconductor wires. That alone smoothed out the idle perfectly but it still ran a little rich.

Next I put back on a stock TBI instead of the the bored one I had. Now it runs better and seems to have more power, especially up hills in OD. I'm thinking that bored TBI just isn't needed on a bone stock motor.

I'm still going to get the injectors cleaned and probably do a few of the CFM tech mods like the salad bowl powercharger and maybe the TBI spacer. I have an adjustable fuel pressure regulator but was in a hurry to have it back together and didn't want to mess with too many things at once. I'll install it sometime soon.
 
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