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Old 02-03-2007, 08:32 PM
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help troubleshooting 89 350 TBI problem?

I've spent countless hours trying to troubleshoot my 89 350TBI engine in my 240,000 mile otherwise incredibly dependable Suburban... The engine runs like crap, ECM says code 44 - lean condition. No power, struggles up hills, runs a little better at higher rpm's, won't climb a hill from a stop hardly at all. At idle in park, the engine revs up and down, like it's gonna stall out,then it picks up, dies down, picks up, etc...at 2-4 second intervals.

I've checked out most things I know to check, but I am not 100% sure that I have ruled out a vacuum leak. A mechanic that put a scanner on it for me said that he thought it could be a leaky intake manifold gasket, or possibly a burnt valve, rather than a computer or sensor problem.

I was told repeatedly to check the fuel pressure, as the pump could be going bad. Spec calls for 9-13 psi. My brand new Actron gauge tells me I have 5 psi. Manual says 9-13 psi. Filter is 2 months old, put in new pump and strainer, cleaned pump ground, still same problem. new pump and old pump are both 5psi after the filter.
EGR is not the problem, checked that, checked PVC valve, looked around for vacuum leaks, nothing I could find thus far.
New plug wires 2 months ago, new cap and rotor, checked compression (all around 150psi), changed throttle body base gasket as possible vacuum leak, O2 sensor replaced, etc... I seem to have plenty of spark.

Took it to my dad's today, who was an old timer mechanic until '81 or so, and he told me just from a test ride alone that it seemed like it was running terribly lean. Took me having the scanner on it to figure that one out!
He is just as baffled by computerized fuel injection as I am. After replacing the fuel pump, we were unsure where to proceed further.
He still says that it behaves like a vacuum leak, except that usually a vacuum leak will cause it to stumble at low rpm's, but take off with power at higher rpm's once it gets going. I have been driving it some, and before the problem got much worse, it would tend to take off once I hit 25 mph or so.
Chuck Sr. was leaning towards a vacuum leak somewhere, or else a stuck intake valve possibility. It backfires sometimes when it is really having trouble getting going.

How can I test the Throttle Position Sensor and MAP sensor? Any clue if either or those could be the problem?

Any othe tips or hints of advice to help me out? I think we are going to look again for vacuum leaks in the morning, and then maybe pull off the valve covers to see if all the valves are opening and closing as they should.

Any other tips, advice, or suggestions are very welcome.

Thanks,

Chuck

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Last edited by Chuck78; 02-03-2007 at 08:46 PM. Reason: adding info
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Old 02-03-2007, 09:00 PM
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Are you sure there is not another filter downstream from the pump that you missed. The spec is 9-15 PSI and your at 5. That is going to drastically reduce your fuel flow into the TB. I'd do an R+R upstream from the TB to verify no obstruction and see if there is a service kit for the regulator.

Last edited by Rick WI; 02-03-2007 at 09:12 PM.
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Old 02-03-2007, 09:13 PM
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All the fuel pumps in the world wont fix a broken fuel pressure regulator spring. Thats a fairly common problem on TBI engines.

I think youve figured out why it doesnt run right(low fuel pressure), now you need to figure out why pressure is so low.

How are you checking fuel pressure?
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Old 02-03-2007, 09:17 PM
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You might consider this... The fuel pressure regulator on those TBI units is on the back right were the fuel lines connect... Its a spring cup type regulator that is preset by the factory and silver soldered in place... The problem is with age the spring rust and over a long period of time it loses tension till it finaly breaks... This spring pressure is what holds the pressure of the fuel against the injectors and builds pressure in the fuel line... Otherwise it will just bypass and basicly give you little or no fuel pressure... You can buy an aftermarket adjustable requlator to replace it or just find a good working unit... (Summit used to have these...?)

I wont say this is your problem but might be...?

Haha... Greg got me by a few seconds...
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Old 02-04-2007, 06:03 AM
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First off, the filter is 2 months old, and the tank looked really clean when I replaced the fuel pump. I am checking the pressure between the engine and the filter (after the filter), by breaking loose the connection between the hard line on the frame and the flexible stainless line (that runs up to the back of the engine), and inserting a tee fitting with a schraeder valve access port on it.
So I am testing the fuel pressure before the regulator, and it is reading 5PSI. The only thing that could be causing that is a plugged filter. I am doubtful, as those filters are rather large, and it would take a lot to restrict the flow that much. Looking at the stream of fuel spraying out of the injectors, it looks like I have a sufficient amount, but I am not really 100% sure what I should be seeing, but I see a healthy cone shaped stream of fuel spraying down onto the throttle plates.

Just for my info, does the fuel flow go through the regulator before it goes through the return loop?
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Old 02-04-2007, 06:10 AM
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I disconnected the lines at the back of the throttle body to pull the unit and replace the tbi base gasket, but I didn't see if there was a screen at the inlet of the TBI. Maybe something could be plugging that up? I still am puzzled by the location of my fuel pressure guage and the low reading with the new and old pump. There's nothing but the line and the filter between the pump and the gauge.

Still not even sure that it's a fuel pressure problem, as it seems to behave a lot like a vacuum leak. Computer says 44 - lean condition.

Thanks guys,

Chuck
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Old 02-04-2007, 07:11 AM
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Quote:
This spring pressure is what holds the pressure of the fuel against the injectors and builds pressure in the fuel line...
Also its on the return side... And you cannot see the spring and there are no screens in the inlet to the TBI...

You could pull the return line and plug it at the TBI and check your line pressure, this would tell you if the regulator is bad but dont run it this way for more than a few seconds or the pump will eat itself...
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Old 02-04-2007, 07:27 AM
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Quote:
I am checking the pressure between the engine and the filter (after the filter), by breaking loose the connection between the hard line on the frame and the flexible stainless line (that runs up to the back of the engine), and inserting a tee fitting with a schraeder valve access port on it.
Aha! So you are checking pressure with engine running and regulator controlling pressure.

You say youve replaced the pump and filter so I assume those are all working good. Still could be that fuel pump isnt getting full 12 volts or has poor ground too but now I really think its the regulator spring.


Pinch off the return line or just dead head the supply line and tell us what the pressure is then.

Also for an expirement try putting 12 volts to terminal G on ALDL that will put 12 volts directly to fuel pump(bypassing relay and oil pressure switch) and force it to stay on all time even when engine isnt running.
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Old 02-04-2007, 07:57 AM
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So my "running" fuel pressure isn't the 9-13psi that the manual specifies? I thought that if I dead headed the pump (return line plugged) that I would generate a massive amount of pressure quickly? Is this how I should be coming up with my fuel pressure reading?

I guess I'm not up how the regulator works. I am thinking in terms of an in-line regulator on a natural gas line, say, going to a furnace (I do HVAC), that reduces pressure, with the full working pressure on the service side of the regulator and the reduced/regulated pressure after that.
So you are saying that this TBI regulator works by regulating the volume of fuel allowed to pass back through the return line? So if my regulator were bad, I would then be allowing a larger volume of fuel to take the path of least resistance through the return line, and the restriction/back pressure caused by the regulator is not enough to build pressure to feed 9PSI+ to the injectors? So the fuel isn't building enough pressure due to the regulator allowing more to flow back to the tank, rather than to the injectors? I think I understand now.

Are you guys then favoring a possible regulator malfunction much more so over the possibility of a vacuum leak or stuck valve? I suppose if I am really running at 5PSI where i should be 9-13/15psi, then this must be it.

I also didn't realize you could ground out A and B on the ALDL and get the check engine light to flash diagnostic code signals... After reading up, I see there are codes for low and high MAP sensor readings, and TPS codes as well. If all else fails, I'll check this to see if I have gotten any other codes spit out of the thing since Tom put his scanner on it two weeks ago.

Last edited by Chuck78; 02-04-2007 at 08:26 AM.
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Old 02-04-2007, 08:44 AM
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http://www.partsamerica.com has a regulator kit for $30, this is what it looks like:
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Old 02-04-2007, 01:55 PM
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Quote:
So you are saying that this TBI regulator works by regulating the volume of fuel allowed to pass back through the return line? So if my regulator were bad, I would then be allowing a larger volume of fuel to take the path of least resistance through the return line, and the restriction/back pressure caused by the regulator is not enough to build pressure to feed 9PSI+ to the injectors? So the fuel isn't building enough pressure due to the regulator allowing more to flow back to the tank, rather than to the injectors? I think I understand now.
EXACTLY!




Yes when its running with regulator controlling things pressure should be 9-13 psi.
If supply line is dead headed (blocked off) or fuel return is blocked off pressure will be slightly higher.
If somehow you manage to put about 20psi on the injectors by mistake they will flow fuel all by themselves, at least on flow tester they did.


That kit you have is probably made by Standard Motor Products and reboxed for a auto parts chain. Its only the diaphragm kit (part # PR131).

I think the kit you need is the entire regulator part number PR113, and its also supplied by GM (dont have access right now to part #) and its a lot more than $30.

http://smpcorp.com/web_app/catalog/publiccatalog.asp



If somehow you end up buying injectors the GM ones have a much better spray pattern.
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Old 02-04-2007, 06:12 PM
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Think you understand now...

Might just check with the dealer and just get a new spring and the regulator diaphram which should not cost to much...

Here is the adjustable regulator
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Old 02-04-2007, 07:08 PM
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Diaphragm solved it all!!! Now that I checked out everything else tune-up and sensor related, the entire truck runs and drives like brand new! Not bad for 240,000 miles! Now if it were that easy to make the body not look like a rustbucket!

The new diaphragm kit came with a shorter, weaker spring, so I used the old one. It wasa good 1-1/4" taller than the replacement. I figured more fuel pressure would make it run stronger, and that I didn't really want to go weaker than stock and lower on fuel pressure (for a 5200 lb truck, no way).
The thing runs really really strong now at all rpm's, best performance I've gotten out of it since I got it 4 years ago. Great running truck, once again, and first major engine issue ever. 3rd transmission though, love those 700R4's... the OD is great however.

The spring was not silver soldered in like someone said I think on the nastyz28.com site (or maybe here), and it was a large coil spring, but there was a part of the regulator housing that did have a stud or bolt with the head of it silver soldered over as to not ever be tampered with.
The TBI is incredibly simple, no real tuning at all. Just put the thing together and go! Set the timing, and that's it! The learning curve was a little frustrating for me, but looking back on it, I understand it really well other than all the mathematics involved in the computer's calculations. It does what it needs to, love how this truck runs. Might yank the engine and entire wiring harness when the body rusts out totally, to save at least the TBI fora 60-66 'burb or panel truck.
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Old 09-14-2013, 11:17 PM
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Quote:
Originally Posted by Chuck78 View Post
I disconnected the lines at the back of the throttle body to pull the unit and replace the tbi base gasket, but I didn't see if there was a screen at the inlet of the TBI. Maybe something could be plugging that up? I still am puzzled by the location of my fuel pressure guage and the low reading with the new and old pump. There's nothing but the line and the filter between the pump and the gauge.

Still not even sure that it's a fuel pressure problem, as it seems to behave a lot like a vacuum leak. Computer says 44 - lean condition.

Thanks guys,

Chuck

My brother (who is no pro mechanic just some gear head knowledge) said, There' wasn't much a ECM(computer) hookup could tell me, I can't get rig to turn over for some damn reason, any who he said, best of luck computers can't tell ya jack. I didn't think this was true but since your seems to pointing you in a circle i wonder if there is even a need to hookup any computer at all. Any ideas, suggestions. Or comments would be gladly appreciated, Hope ya get ur run running, please keep posted your effots.
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Old 09-15-2013, 06:32 AM
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