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86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

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Old 08-16-2012, 02:47 PM   #1 (permalink)
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Fuel pump failure vs fuel pressure regulator failure

I think I narrowed my engine performance issue down to a lack of fuel pressure but am wondering how to determine if it is a faulty fuel pressure regulator or a bad fuel pump. Any insight? I have yet to put a pressure gauge on my fuel line but if i did it would be where the cold start injector is unless anyone has a suggestions.

Please give some comments on how to narrow this problem down besides throwing money at it.
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Old 08-16-2012, 03:25 PM   #2 (permalink)
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1. Throwing money at it won't work anyway, so good choice!

2. The "factory" fuel gauge (the one shown in the FSM) goes on the CSI, because the fitting on the gauge is the "banjo." I've never actually seen one like that. But if you get an Actron CP7838 http://www.actron.com/product_detail.php?pid=16173, it comes with a 12mm x 1.25 banjo "bolt" to Schrader adapter. I've never used it myself, but I believe that bolt is too large for the CSI, and you'll go on the connection near the fuel pressure regulator. (Actron carries different size bolts, but if you buy this at Autozone you get what comes in the kit.)

3. Once you determine that low fuel pressure IS the case (what reading are you looking for?), you can just pull the return line from the regulator. If fuel is pouring out, you've got a bad regulator (it's regulating too low). If no fuel comes out, its the pump (because it can't get the pressure up to where the regulator works.) Or maybe its the filter. Or maybe a crimped fuel line. Or the sock ....
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Old 08-16-2012, 04:16 PM   #3 (permalink)
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The fuel pressure regulator most likely will not be it, as scope103 said above me, just take the return line off and see if there is fuel, if there is nothing, the fuel pump is bad.
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Old 08-16-2012, 06:45 PM   #4 (permalink)
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Your local oreilly's or autozone carry the actron kit.

You can borrow it on the loan a tool program.

The kit does have the correct fitting for the CSI banjo.


A couple points to watch out for.


The copper washers in the kit suck. They will leak at a moderate tightening of teh banjo bolt.

The banjo fitting is cheap chinese aluminum.

If you try to tighten it a little more to stop the leak, it WILL shear...really at a very low torque.

Hylomar does a good job of temporarily sealing your old crush washers (maybe even the crappy copper ones in the kit).


So, get a new set of crush washers from the dealer....they will charge you $2-3 apiece.... which is too high, but still this is likely to be one of your cheapest dealer parts bills.

Use your old washers, optionally with a light smear of hylomar, when attaching the banjo adapter fitting.

DO NOT OVERTIGHTEN...if you do you will be returning the autozone kit and borrowing the oreilly kit...ahem.

Look at your FSM for values and test procedures for the pump and pressure regulator.

Don;t take the gauge as gospel...it is a chinese gauge. Look for trends and basic information. If it says 40 psi...it might be 40, 38, 42 or some other pressure altogether.


The above holds true of most affordable instruments.

Lastly, if you want to purchase your own pressure tester...

http://cgi.ebay.com/ws/eBayISAPI.dll...ht_5351wt_1037

I oddly enough yesterday placed an offer of $40 on one of these and the seller accepted.

I've been casually looking for a little while, and don;t think you can get a much better deal.
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Old 08-16-2012, 07:26 PM   #5 (permalink)
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its is very rare for a fuel presure regulator to go bad. Its can and does happen, but its not a common problem. Chances are its your fuel pump thats getting weak.

When was the last time you replaced the fuel filter ?, a plugged fuel filter will cause the problems your having.
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Old 08-17-2012, 07:10 AM   #6 (permalink)
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The reason that I was thinking that it was the fuel pressure regulator was because the truck runs better if i pinch the return line to the tank from the regulator. Cylinder 4 doesn't seem to fire when at idle but works otherwise... Here is a link to my previous post discussing the problems and why i came to the conclusion it was a fuel system issue.

http://www.yotatech.com/f116/1987-22...n-idle-257334/
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Old 08-17-2012, 07:13 AM   #7 (permalink)
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BTW, I replaced my fuel filter 10,000 miles ago when I installed a new long block.
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Old 08-17-2012, 08:17 AM   #8 (permalink)
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Pinching off the return line will raise the fuel pressure with a correctly working FPR. So, if cylinder #4 was running lean (vacuum leak? clogged injector?), pushing up the fuel pressure will make everything run rich for a few seconds.

So how do you figure it out? You were right the first time; you need a fuel pressure gauge. Don't worry about 2-3 psi one way or the other (the spec isn't that precise anyway), you're looking for a big difference.
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Old 08-17-2012, 08:50 AM   #9 (permalink)
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I've already swapped injectors and the problem stayed with cylinder #4, if you read my other post it says what i have looked at. Ive eliminated electrical, spark and injector issues, so the only possible issue left is a clog in the fuel rail or a pressure drop which is letting the other cylinders get the proper amount of fuel but not cylinder #4, unless im missing something.
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Old 08-17-2012, 08:50 AM
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