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TBI running rich*update post 23*fix post45*

R

RedBrute

1/2 ton status
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Location
Southern tier New York
I have been putting up with this all winter and decided to tackle the problem this weekend
Engine is a TBI 350 with Thorley headers, Magnaflow cat, all else stock
Tuneup done last summer, cap, rotor, plugs/wires and coil. Timing set at 4*BTDC. All vacuum hoses vac checked for leaks, none found. Stock ACDelco fuel pump and filter along with new NAPA sending unit/fuel line assy.
The problem is when accelerating from a start and using just enough throttle to get the truck moving, the engine starts to bog and "load up" like a carb with a choke hanging. If I give slightly more throttle it appears to clear up and drives OK.
If cruising at say 35mph and just enough throttle to maintain speed it'll start to load up and"buck" until the throttle is depressed, it then clears up.
Outwardly the evidence is a black sooty tailpipe as well as slightly sooty plugs. Truck drives fine other than the bogging/bucking occasionaly and does not give a code.
The O2 sensor is a Bosch that is about 4 years old.
I have an old OTC monitor 2000 hand held diagnostic tool that allows me to scan thru various "modes" and look at IAC,TPS, BLM, O2 volts etc, as well as LED flags that indicate closed loop, open loop,TCC etc.
I went for a test drive and checked a number of things. Most important I think is the BLM, never gets above 108 while cruising at 65mph and the Integrator varies from 113 to 129. The O2 volts bounce all over the place, from .040 to .800.
I can get the BLM to increase but only when accelerating.
I tried a completely different TBI this weekend thinking my 250k mile original unit was tired. Other than a slightly lower idle it didn't change a thing.
I'm stumped, I have a lot of info from the diagnostic but don't have the experience to know what is right and what is wrong.
Any suggestions???
 
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Just a few things off the top of my head.............
Sometimes having driveability problems with no code set means the dist module could be bad. Also could be the magnetic pick up coil.

What does the voltage look like from the TPS? Is is smooth and steady .50V to aprox 5V as you accelerate? It should be. If the voltage jumps around when the throttle is depressed or held at one position the ECM interpets the voltage spikes as acceleration and richens up the fuel mixture.

Does your scanner read 02 cross counts? If so check cross counts at idle. If the counts move slowly or stop at idle. the 02 sensor may be bad.

When you set the timing did you disconnect the bypass wire?
 
TPS is smooth, took it for a test drive and watched the volts as my son drove. At first I was convinced it was tps.

Yes my scanner reads cross counts. Same test drive I "logged" data by writing things down . Crosscounts at idle flip from 2 or so up to 20, cruising at 65mph they run in the low teens to mid 20's.

I read a post on here that called out Bosch O2 sensors as junk so I swapped in a new ACDelco unit yesterday. Slight improvement in throttle response, O2 sensor volts don't jump from one extreme to the other (.010v to .800v) like the Bosch did.

I've never had the dist out, IE it's original. Yes I had the wire disconnected when I set the timing. Maybe it's time to re-man the dist.

Also, truck runs smooth while in open loop, no bogging. as soon as it goes closed loop the problem appears.
Thanks for your help...what next, dist?
 
Distributor problem shouldn't be open/closed loop noticeable. Should be consistent while it's running.

O2 sensor volts SHOULD bounce, if they are steady something is wrong. Too complex to write out quickly, we'll leave it at that. :)
 
The O2 does vary quite a bit, I understand that. I was trying(poorly) to say the AC seems to have a tighter range than the 4 year old Bosch. I've been trying to educate myself by reading old posts on here and alot on 3rdgen to see if the problem is in the TBI/sensor area or maybe spark/fuel delivery related. It seems to me the TBI is spraying too much fuel and the 'puter is telling it to back off (BLM 108) but it's just not getting lean enough.
 
Yeah, it is possible the injectors have problems, but I really don't know much about the TBI injectors...there are seals and what not, apparently those can go bad and the injectors leak? I would think a pressure test would reveal if they leak, but beyond that, whose to say the injectors internally aren't bad?

Do you get a good cone shaped spray of fuel? I wonder if with the vehicle cold, air cleaner removed, you have someone turn the key to run (not start) so that the system pressurizes, if the seals or injectors are bad, if you'd see fuel dribbling out. Not so scientific as a fuel pressure test, but I would think any sort of leaking fuel indicates a problem, IF that's a way TBI injectors can go bad.

With the system trying to correct for a rich condition, it is possible it's able to correct enough to keep the O2 swinging lean/rich, but if it's leaking somehow, obviously there is a lot more correction going on than should be necessary.

Have you been able to find the chart that correlates MAP voltage to vacuum? (I think it's one of the factory manual troubleshooting pages, may be TPI) If the MAP is reading incorrectly, it will send the system rich or lean, depending. I believe a quick (not authoritative) GM test for the MAP is to unplug it and see if the vehicle runs better. I'd still want to compare my vacuum readings against MAP voltage to ensure something else isn't going on that's affecting vacuum, etc.
 
I have watched the injectors spray at idle, cold and at operating temp. The spray pattern is cone shaped (nice cones hee hee). It appears the spray hits the top of the throttle plates and puddles ever so slightly before getting sucked in past the plate edges. Problem is this is the only TBI I've worked on, and I may be looking at a problem and not recognize it. I think tomorrow I'll try the map test. Using my hand held I can look at MAP values both volt output and "pressure" seen by the 'puter. Thanks for your help.
 
Thats cool that the hand held will do both. Even if it doesn't answer the question as to why this is happening, it will eliminate a few things.
 
I would definitely check the fuel pressure. If there is a restriction in the return line it could raise the pressure and make it rich.
 
I have a theory that my TBI is running rich due to an exhaust leak at the outlet of the manifold. If fresh air is being sucked in through a leak right near the O2 sensor, it will think it is too lean and add fuel 'till it gets the "correct" O2 switching. End result: "correct" O2 signal, but running rich.
 
BGKYK5 said:
I have a theory that my TBI is running rich due to an exhaust leak at the outlet of the manifold. If fresh air is being sucked in through a leak right near the O2 sensor, it will think it is too lean and add fuel 'till it gets the "correct" O2 switching. End result: "correct" O2 signal, but running rich.
Click to expand...

Yes, an exhaust leak can effect the air/fuel ratio in closed loop mode. In that case the BLMs would be high as the ECU added fuel. RB's BLMs are at the minimum value at a steady 65 which means the ECU is subtracting as much fuel as it can. The fact that the integrator makes it to 129 indicates it is almost able to correct the mixture at a BLM of 108. He needs to log the data to see what it is doing at the point that he is having the problem.
 
The only thing I can think of is what Creek said. Check the fuel pressure. Inspect the return line for kinks, ECT.
When you swapped the TBs did you use a diferent regulator?
 
I do need to check the FP, I don't have a gauge just yet. This last weekend I opened up the pressure return line to the tank. I ran the flow into a gas can and has good flow comming from it. For the portion back to tank I blew air into it and could hear it bubble into the tank so I believe right now the lines are OK,
Yes, when I swapped the TBI it was a complete unit.


Also I retract my earlier statement that it runs fine in open loop... it "bogged" on me twice yesterday both times it was stone cold so whatever it is it happens all the time warmed up and not as often cold. I appreciate the help guys, keep 'em coming...
 
Dorian, I have a MAP/VOLTS graph in the manual for my hand held, not sure how accurate it is but the readings I get match up pretty well. If I can believe this graph I do not have a problem with my map sensor...

Map-volts1.jpg
 
I reread your first post. No mention of the coolant temp sensor. Is it reading the right engine temp?
 
Good idea, easy check, coolant temp is an easy deal, especially if now it is not closed loop dependant.

You now know pressure is being translated to voltage correctly, is engine vacuum where it should be?
 
Yes, I drive to work and back home literally holding the hand held in one hand and steering wheel in the other. I can watch the coolant temp rise from 62* (last nite) and settle out at 180* while cruising.

You don't suppose that the 'puter itself may have a problem? Maybe whatever part of the circuit board "fires" the injectors maynot be able to shut off fast enough due to a failing component on the board? I have an '88 truck in the driveway. It is the "donor" truck to turn my sons '79 into a TBI motor. I could try that 'puter??? It is a manual tranny truck if that matters.
 
I'm not sure if engine vac is "where it should be"...I know cruising at 65mph the TPS is1.64v, map is 76KPA, RPM 1725. I'm off to work right now, I'll pay attention to this and get back to this in about 12 hours. (long shift)
 
I watched the MAP while driving and the voltage values match up to the KPA on the chart I posted nearly perfect... at 60KPA the MAP voltage was 2.74, at 30KPA the MAP volts are1.16. I also checked the "new" TPS. I put it in drive, held down the brake and slowly depressed the gas pedal. TPS voltage increased slowly with no sudden change so I'm stumped. CTS is good TPS is good MAP is good. Tomorrow morning I'll check fuel pressure and post up readings
 
I guess I should explain better. :)

What does your idle MAP voltage correlate to in Hg? I just want to make sure you don't have a vacuum leak of some type. Didn't sound like it with 20+ at one point, but idle is where you will easily see something like that.

As to CTS, you are saying it started out very close to ambient temp, then rose to 180 and stayed pretty constant? That should be ok if so.

You can swap the computer, as long as they both have numbers ending in '7747. Just need to make sure if the problem goes away, swapping the old one back in duplicates prior performance. I've had a connection on one of the ECM wires before, it screwed with me for quite awhile lol.

Computers are typically NOT the problem, but of course they can and do fail, sometimes in weird ways. I like to throw out a guesstimate of 90% of the time or more they aren't. I've seen that figure thrown out in regards to the repair market for ECM's...90% of them that get tested after a replacement, aren't bad. Or so I was told.
 
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