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Username Post: TBI 7.4 fuel pressure, pump, tester questions        (Topic#182063)
someotherguy 
Moderator
Posts: 25615
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
02-12-08 06:18 PM - Post#1364777    

So my buddy ran his '89 7.4 out of gas today, or at least he thinks he did - his gauge doesn't work. He put some fuel in and it tries to start, may run for a second or two, roughly, then stall. In case he'd put some junk through the filter, we threw a new filter on it, no change. The old filter (which was actually fairly recent) was very hard to blow through.

I didn't have a pressure test kit so I figured today would be a great day to buy one. Picked up an ATD #5567 which says plainly on the label "not" for GM TBI systems. Seems weird to me but I guess as a liability/safety issue since it doesn't have the fittings to thread right onto the hard lines. That's my best guess, any opinions on that?

Well, the Tee fitting with it has barbs for 1/4, 5/16, and 3/8 hose - so we shoved some 3/8 line over the hard lines, onto the barb, and hooked it up, and with the filter removed from the system. Whether it's key on/engine off, or starting, it struggled to build even 10 PSI. Spec for a TBI 7.4 is 26-32 PSI, so it sure looks like we have a problem here. Just to confirm the gauge I slapped it on my '98 7.4 (which admittedly is a little different since it's not Teed in; just uses the single hose off the test port) and it showed 60 PSI as expected.

So before my buddy drops his tank and buys a pump and sock, what do y'all think? I suppose I trust the tester, but would like to hear some opinions. I'm surprised that with the spec of 26-32 that it would even try to start with 10 PSI but I guess it's been my experience that the TBI engines aren't nearly as sensitive to pressre as the Vortecs.

Edit - here's the tester I bought. http://www.sjdiscounttools.com/atd-5567.html (CLAIMS to be made in the USA; I sure wish it had a pressure relief valve for convenience.)

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects


Edited by someotherguy on 02-12-08 06:20 PM. Reason for edit: No reason given.

 
Low priced Genuine GM Auto Parts
Boog 
Super Senior Member
Posts: 3535
Boog
Loc: Central Arkansas
Reg: 04-28-02
02-12-08 07:26 PM - Post#1364836    
    In response to someotherguy

Compared to a small block TBI that 26-32 for the 7 lt does sound high but if the book specs it...Well you know the pump is running and the pressure is no more than 10 and the gauge registers correctly on your truck. Sounds like time to lift the bed or drop the tank to me.
Fuel pressure regulator?
Boog
69 Chevy step
06 GMC ccsb Z71
00 GMC excab Z71
All GM...'nuff said!
JR Nation


 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2993

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
02-12-08 07:32 PM - Post#1364840    
    In response to someotherguy

Hi Richard

The 1994 and newer big block TBIs run at a higher pressure. In 1994 the 7.4's use a small injector with much higher fuel pressures of around 26-32psi. But a 1989 should have the same pressure as the small block engines with a spec of 9 to 13 PSI with the truck running.

You must T-in the gauge to test the pressure.

Here is the standard reply I make in regard the testing TBI fuel pressure:
Here is some info about fuel pressure testing I saved from a previous reply I made to a similar post. Hope the links still work.

C/K truck TBI systems do not have a fuel pressure test port.

The fuel pressure is tested by using fittings to “T” in a pressure gauge. I like to T in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If pinching off the return line, using care not to damage the return line hose, makes a low pressure reading surge above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.


Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.


I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.

http://www.sjdiscounttools.com/ta37650.html

http://www.actron.com/product_detail.php?pid=16174

http://www.actron.com/product_detail.php?pid=16175

http://www.cfm-tech.com/catalog/fuel_-_cfm-te ch_fu...

Another addition to the list:
http://www.tradervar.com/cgi-bin/store/agora.cgi?c...



For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
someotherguy 
Moderator
Posts: 25615
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
02-12-08 08:12 PM - Post#1364874    
    In response to Chevytech

Okay, thanks for that - I had no idea they would have used different pressures over the range of TBI 7.4's. Crazy.

So it sounds like it's in spec, then; now I have to figure out what's wrong with the piece of junk.

It started doing this immediately after he "thinks" he ran out of gas. I'm starting to wonder if maybe it's an ignition problem and he just assumed it was out of gas. He left it idling when he went inside somewhere, came out and it had stalled. When he tried to start it, it was hard to start, ran rough, and stalled out - so he put gas in it, and it didn't really get much better.

Now that we're straightened out on the fuel pressure spec (thanks again!), I'm starting to wonder if maybe his ignition module didn't croak. I'll probably have him pull that tomorrow and have it tested, and since I don't trust the test 100%, probably grab a spare used module just to see if it acts any differently before spending the money on a new one.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects


 
Bishop 
Frequent Contributor
Posts: 1488
Bishop
Age: 27
Loc: DMV Area
Reg: 01-06-07
02-12-08 11:00 PM - Post#1365019    
    In response to someotherguy

Well you already took my guess Richard i was going to say the ignition module as well. I had an 86 c1500 and it would go through ignition modules all the time i hated that dang dizzy. But i got my 98 now so i'm happy!
Not only so, but we also rejoice in our sufferings, because we know that suffering produces perseverance
Romans 5:3


 
someotherguy 
Moderator
Posts: 25615
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
02-13-08 04:14 PM - Post#1365447    
    In response to Bishop

Well, no luck so far. His module tested good at the auto parts store, and just in case the test wasn't catching a bad module, I loaned him a "genuine" GM one I had in my toolbox, same results.

It will start and run if he feathers the gas pedal but it runs very rough. When you let off the pedal it will stall immediately.

Any ideas? It's kind of frustrating for me to try to help him because he's across town; in a couple of weeks we'll be set up in a new shop where we can work on it but he needs it running right before then.

Fuel pressure should be good, ignition module should be good, truck was running fine with lots of power until it stalled yesterday while he was away from it for a few minutes.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects


 
Boog 
Super Senior Member
Posts: 3535
Boog
Loc: Central Arkansas
Reg: 04-28-02
02-13-08 05:33 PM - Post#1365496    
    In response to someotherguy

Rich, forgive me if I missed it but I didn't see any mention of scanning it for codes? Surely there's something there.
Boog
69 Chevy step
06 GMC ccsb Z71
00 GMC excab Z71
All GM...'nuff said!
JR Nation


 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2993

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
02-13-08 07:00 PM - Post#1365574    
    In response to someotherguy

Check and see if the EGR valve is stuck open.
For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
someotherguy 
Moderator
Posts: 25615
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
10-04-08 10:12 AM - Post#1535118    
    In response to Chevytech

Wow; I can't believe I failed to respond back with what was going on with this truck. I found it while searching for good links to use in the FAQ.

Anyway, he got frustrated with it and took it to a mechanic buddy of ours and they spent day after day battling with it, threw a kit in the throttle body, started throwing sensors at it, just about anything you could imagine, they tried it.

And in the end, it was something that has bitten several of us before. Although the fuel pressure test led us to believe all was well, the short section of hose attaching the pump to the hardline of the sending unit had a small hole in it.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects


 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2993

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
10-04-08 06:29 PM - Post#1535365    
    In response to someotherguy

I loose track of threads way to often. Sometimes I am looking for a thread I replied to on the wrong message board as I belong to multiple Chevy boards.

On this board, if there is a way of seeing what post I have replied to, it does not work with high security settings. I am sure I have lost track on posts I had replied to.

For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
someotherguy 
Moderator
Posts: 25615
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
10-04-08 06:35 PM - Post#1535372    
    In response to Chevytech

Click on your own username, then select "Find posts by this user" - it seems to be limited to the past 500, but it'll show all your recent posts. This would be instead of selecting "Find topics by this user" since that would just show topics that you started, instead of ones that were started by others, and you replied to.

This is part of the "Search" function, and as such, can be a little quirky. If you pull up the list of results and just start clicking on them, when you hit the "Back" button, it may give you a "Page expired" error. I work around this by not directly clicking the results. I right-click them and "Open in new window" then I can close that window and the original listing is still intact. Hope that makes sense; my brain isn't at its sharpest at this moment.

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects


 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2993

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
10-04-08 07:04 PM - Post#1535389    
    In response to someotherguy

Thanks, it makes a lot of sense. I did not think of opening the posts in a new window so I can keep going down the list.
For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
Low priced Genuine GM Auto Parts
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