Need Help: 350 TBI Running Rich/Bogging

[5Tractorguy]

I've been chasing this problem ever since the truck went on the road 6yrs ago. Hoping someone on here can help.

Truck is an '88 V30 1-ton 4wd dump truck.
(Yes, I know it's the wrong year for this forum, but it's more relative to this section than mid 90's GM's).

It has always had a power problem pulling, driving, consuming way too much gas. Some times worse than others. The 350 was completely rebuilt when the truck was restored. And the air injection system was removed but the engine wiring harness wasn't changed (could be a problem)?

Anyways, I've been through 3 different computers, it's currently running best on one from an '87.
Injectors are re-manned
Fuel pressure regulator is rebuilt
Fuel pressure is good
Ignition coil is new
New cap and rotor
New Spark plugs
Timing is set to about 2-3deg (set with advance wire unplugged).
I think at this point every sensor has been replaced (maybe one went bad again)
Vacuum lines are where they should be
New EGR valve
New O2 sensor

I just did the TPS again on Friday (you could wiggle the plug and it would hunt so I figured the sensor) and now it's running even more bogged. Using gas to the point that I watch that gauge more than the speedo.

When I did the O2 sensor about 2 months ago it woke right up and ran great. Bunch of power and then it slowly started the decline again. I'm wondering if since the air injection system was removed it's not reading something right and in turn calling for too much fuel resulting in fowling the O2 sensor??

I've been getting real close to converting this thing to a carburetor and doing away with the TBI, just want my power back.


Any help/direction would be greatly appreciated folks!

[Dayton601]

What is your current fuel mileage, most got around 16 to 18 mpg , i had a 91 did several thing to try to improve fuel mileage, I started at 15 and finished at 21 to 22

first I would check fuel pressure,
then spark plug gap and maybe go up a hotter setting, I did run the platinum plugs, not sure that did anything,
Also reading that you changed the the O2 sensor, did you go with factory. the aftermarket stuff is junk, I have heard people go through several autopart brands before finding a good one,
I would add throttle body spacer and fuel injector spacer, two different things, the spray pattern looks like a cone,
then I would add a hotter (super coil) coil, seems that's were I picked up my best fuel improvement I kept the stock coil mounted on the intake, hooked up my super coil, drove 300+ miles Houston to Dallas , then i would hook back up my factory coil, drive it back, did this several times to confirm, I say this because some will say coil will make no difference, each time there was at least a 4 to 6 mile difference, perhaps the octane grade might have an effect, nothing beat the coil, even running with windows down, A/C off and tailgate down,
Also was running 8.8 wires, better insulated, once i had just a cheap set of autozone wires, and I happened to open the hood when it was really dark, it looked like Christmas lights, after that I made sure i bought good wires,
that just my 2 1/2 cents worth

[Desert1957]

Check the MAP Sensor, Is the tailpipe excessively sooty? Black smoke?

If so , locate the map sensor and pull the vacuum line from the sensor, engine should run fuel rich and have a noticeable change the way it sounds.

Make sure it has FULL vacuum at idle , check it with a gauge if needed. This is a CRITICAL sensor to the efficient way a TBI system runs.

[Desert1957]

Here is where the Map sensor is on my 87 R10 5.7

[RUN GMC]

I recently had the same problem on my 87 replaced everything except the spark plugs wires which in hind sight I should have done first but since they were not really old (14 months).....so I decided to replace the spark plug again and I saw only 3 spark plugs were fouled somewhat compared to the others (and I had laid them out in order to know what cylinders they came from)....after replacing plugs I started up the engine and noticed I had the same bog down and a slight miss....at this point I removed and replaced the 3 spark plug wires for cylinders that had the fouled plugs.....upon recranking it up I could tell it sounded better and it pulled better but I also read that it takes 40 cranks cycles to get the ecu to dial in the parameters to make it run really great again.....hope this gives you food for thought.........

[5Tractorguy]

Sorry for the delayed reply guys, had vacation last week and a bit under the weather this week...
Thanks for all the input! I did the coolant temp sensor last week and it seemed to help. I left the battery disconnected while changing it to reset the ECM. It's pickup a bit more juice, still not where it should be, but not nearly as boggy to drive.

Originally Posted by Dayton601

What is your current fuel mileage, most got around 16 to 18 mpg , i had a 91 did several thing to try to improve fuel mileage, I started at 15 and finished at 21 to 22

first I would check fuel pressure,
then spark plug gap and maybe go up a hotter setting, I did run the platinum plugs, not sure that did anything,
Also reading that you changed the the O2 sensor, did you go with factory. the aftermarket stuff is junk, I have heard people go through several autopart brands before finding a good one,
I would add throttle body spacer and fuel injector spacer, two different things, the spray pattern looks like a cone,
then I would add a hotter (super coil) coil, seems that's were I picked up my best fuel improvement I kept the stock coil mounted on the intake, hooked up my super coil, drove 300+ miles Houston to Dallas , then i would hook back up my factory coil, drive it back, did this several times to confirm, I say this because some will say coil will make no difference, each time there was at least a 4 to 6 mile difference, perhaps the octane grade might have an effect, nothing beat the coil, even running with windows down, A/C off and tailgate down,
Also was running 8.8 wires, better insulated, once i had just a cheap set of autozone wires, and I happened to open the hood when it was really dark, it looked like Christmas lights, after that I made sure i bought good wires,
that just my 2 1/2 cents worth

My millage averages about 10-10.5MPG right now. When it boggs badly, usually 9.5.
O2 sensor was changed with an Acdelco replacement. I've been trying to replace everything with factory name brand hoping it will last longer. Hotter coil is a good idea. I remember changing that a couple years ago with a new one and the difference was incredible when the new coil went in. I could try to find a hotter one.

I'll check the wires too, that's a good idea at night. I think I'm running the heavier set that you can get through NAPA if I'm not mistaking.

Originally Posted by Desert1957

Check the MAP Sensor, Is the tailpipe excessively sooty? Black smoke?

If so , locate the map sensor and pull the vacuum line from the sensor, engine should run fuel rich and have a noticeable change the way it sounds.

Make sure it has FULL vacuum at idle , check it with a gauge if needed. This is a CRITICAL sensor to the efficient way a TBI system runs.

MAP sensor was done about 3 weeks ago. Definitely made a difference, replaced with ACDelco. That is one thing I have to do though is check the vacuum and see what it reads. Along with check all the lines for cracks again. Would a bad charcoal canister have anything to do with the way it runs? Do they ever go bad?

Originally Posted by RUN GMC

I recently had the same problem on my 87 replaced everything except the spark plugs wires which in hind sight I should have done first but since they were not really old (14 months).....so I decided to replace the spark plug again and I saw only 3 spark plugs were fouled somewhat compared to the others (and I had laid them out in order to know what cylinders they came from)....after replacing plugs I started up the engine and noticed I had the same bog down and a slight miss....at this point I removed and replaced the 3 spark plug wires for cylinders that had the fouled plugs.....upon recranking it up I could tell it sounded better and it pulled better but I also read that it takes 40 cranks cycles to get the ecu to dial in the parameters to make it run really great again.....hope this gives you food for thought.........

Thanks! That it does. I usually replace the plugs every year and the wires every 2. I keep a log on what gets done to the truck so maybe it's time to check that and replace again.

[CSHADES]

The first thing I would check on this before throwing more parts at it is the actual vacuum reading. Do you have 18 inches of vacuum at idle or at least close to that and what is the reading at about 2500 rpm? The MAP sensor is one of the main inputs as far as fuel delivery on these old obd1 systems, the ecm will ignore the o2 sensor readings if it thinks you are under a heavy load/low vacuum situation i.e. wot or close. If the vacuum is low I would look at either a timing chain problem or maybe a outer ring slippage on the balancer that make the timing look correct when it isn't.

[DieselSJ]

You think 10mpg is bad for that truck? A 1-ton 4 wheel drive dump truck? That is a lot of truck for a stock TBI 350. You really won't do much better than 9-10.

OK, with the engine running, point your timing light at the bottom of the injectors. You should see a nice even cone pattern with no dripping. Also, reman injectors for those are garbage. For not much more $$ you can get new. Also are you sure you got injectors for a small block and not for a big block? It is easy to screw up the o-rings around the injectors and they will leak.

You said fuel pressure is "good". What is the actual PSI and where did you measure it?

Check engine light on? Failed sensor will almost always trip a CEL. If you are as rich as you say then it should trigger a CEL for that.

Make sure you got the correct EGR. I know for my 87 there are 2 versions and they operate differently and they are not interchangeable.

[5Tractorguy]

Originally Posted by CSHADES

The first thing I would check on this before throwing more parts at it is the actual vacuum reading. Do you have 18 inches of vacuum at idle or at least close to that and what is the reading at about 2500 rpm? The MAP sensor is one of the main inputs as far as fuel delivery on these old obd1 systems, the ecm will ignore the o2 sensor readings if it thinks you are under a heavy load/low vacuum situation i.e. wot or close. If the vacuum is low I would look at either a timing chain problem or maybe a outer ring slippage on the balancer that make the timing look correct when it isn't.

Sounds good. Guess I'll have to pick up a vacuum gauge and see what the deal is.

Originally Posted by DieselSJ

You think 10mpg is bad for that truck? A 1-ton 4 wheel drive dump truck? That is a lot of truck for a stock TBI 350. You really won't do much better than 9-10.

OK, with the engine running, point your timing light at the bottom of the injectors. You should see a nice even cone pattern with no dripping. Also, reman injectors for those are garbage. For not much more $$ you can get new. Also are you sure you got injectors for a small block and not for a big block? It is easy to screw up the o-rings around the injectors and they will leak.

You said fuel pressure is "good". What is the actual PSI and where did you measure it?

Check engine light on? Failed sensor will almost always trip a CEL. If you are as rich as you say then it should trigger a CEL for that.

Make sure you got the correct EGR. I know for my 87 there are 2 versions and they operate differently and they are not interchangeable.

No, I'm not worried about the MPG here. I don't mind getting 9-10 I just want my "umf" back!

I will have to check the injector spray again with the timing light. I remember doing that before and seeing the cone, just need to look for the dripping.

How can I tell the difference between injectors?

Well, stupid assumption on my part... I'm just assuming it's good because it runs the same on either tank (has saddle tanks). And the pumps were replaced about 4-5 years ago. I should pickup a gauge though.

Negetory in the check engine light. I tried plugging in the code reader too and it won't read anything on the computer.

[5Tractorguy]

Originally Posted by CSHADES

The first thing I would check on this before throwing more parts at it is the actual vacuum reading. Do you have 18 inches of vacuum at idle or at least close to that and what is the reading at about 2500 rpm? The MAP sensor is one of the main inputs as far as fuel delivery on these old obd1 systems, the ecm will ignore the o2 sensor readings if it thinks you are under a heavy load/low vacuum situation i.e. wot or close. If the vacuum is low I would look at either a timing chain problem or maybe a outer ring slippage on the balancer that make the timing look correct when it isn't.

Finally got around to checking the vacuum. Gauge reads 21-22in at idle and right up through 2500rpm and beyond. Is that okay? Should it be varying more throughout the throttle range?

Going to be checking the ignition timing and injectors next.

Edit: This was tested at a stand-still with the truck running.

[CSHADES]

are you sure your vacuum gauge is correct? around 18" at idle and a bit less at 2500 rpm is normal I don't ever think I have seen one that high before.