outdoorjunky

First of all I've been having some issues with my fuel injection, pretty sure I have one or more leaking fuel injectors. I get 40psi fuel pressure at the rail when I jump the FP at the diagnostic port or when the key is in the on position but when the truck is running it drops to 28psi and the pressure leaks off within a couple of seconds when the engine is not operating. I pinched the soft return line from the FPR and shut off the engine and pressure held for quite a while. That said my fuel pump is brand new (Denso), the FPR is new (inter motor brand) the fuel filter is new and routed correctly, and the fuel dampener is deleted from the rail, I also can't detect and leaking fuel between the plenum and the tank I even crawled under there and raced along the hard lines but saw no signs of leakage. So I'm having a question as to whats causing the pressure to leak off ... seems like it's something between the soft return line and the tank right? or pressure wouldn't have held when I pinched off the line?

Second related issue: today I noticed my EFI relay (driver side fuse/relay block # 1) was getting really hot. I got home for lunch and pulled it and resistance tested it and it's nearing the high side of spec 75.6 ohm spec is 60--80. I then tested the voltage at the block terminal and it gets 12.30 volts with the key in the on position but if I crank the engine that drops drastically to 10.2 volts. I'm just wondering what is an acceptable voltage drop when cranking is occurring? I noticed it drops to around 10 volts and takes about 10 seconds to recover back to 12.3 volts. my battery voltage is 12.49v. I've been not only having a random misfire but also issues with cranking in the mornings ... this is partly because of my cold start wiring (my harness is toast and needs to be replaced) but also it seems like the EFI relay may be coming in to play in this issue as well.

So the main questions I have here is what is an acceptable voltage drop for the EFI relay circuit? and is 10v bellow that threshold? Secondly what might be the cause of the fuel pressure leaking off so fast?

Thanks in advance!

outdoorjunky

Well, this is embarrassing. I found the answers I was looking for in the FSM of all places (*Sigh) my bad for posting.

scope103

Gee, a big part of why ANY of us post, is to try to provide a little help to those who follow us.

So what was the answer?

(Just to put in my bid: it sure sounded like your FPR was not working correctly.)

outdoorjunky

haha alright point taken I was just trying to avoid the "use the search get an FSM" crowd.

Fuel pressure wise 28 psi is within spec ... spec is 27-31psi at idle. Actual readings on my pressure gauge when the FP is jumped or the key is turned to the on position are closer to 38psi which is also within spec.

The FPR being the issue seems so obvious but I had the pressure issue, replaced the FPR that was on my truck with a new one, and still have the same bleeding off of pressure after. That said maybe both parts are/were bad, sure seems like it since nothing is stopping the fuel from exiting the FPR unless I pinch the return hose off.

The FSM answered my voltage question about the EFI relay, in the EFI troubleshooting section it shows that good voltage for the EFI relay is 10-14v so if mine is at 10.2v cranking then the circuit is still in spec.

About the FPR, last time I bought the Intermotor brand FPR I guess they sell them at rock auto but I got mine at one of the local parts stores, same part number. Got any recommendations on a better option? OEM is discontinued as I'm sure you know.

scope103

Some basics. The fuel pressure at idle from the '93 manual is 33-37psi. http://personal.utulsa.edu/~nathan-b...28fuelpump.pdf If you have an '86 manual, you should stick with that.

The most common reason for bleeding off of pressure is a weak check valve in the fuel pump. (It's leaking back through the pump, so clamping the return line shouldn't affect that.) Bleeding off is not really a big deal; at most it means you'll have to crank it a little more when starting right after shutting it off. The spec is 21 psi or more after 5 minutes, so even with a brand new truck, you're starting with no pressure in the rail after sitting overnight.

The relays should be slightly warm to the touch when they been "on" all day, so one that feels "hot" to the touch is trying to fail. RockAuto has them for $6, so just replace it. DO finger-test it after the replacement; if the plug contacts are-high resistance they can heat up even a new relay.

I have an Intermotor FPR in my truck, and it works fine.

12.49 battery voltage is a little low (12.6 is about in the middle, but it's temperature dependent, so 12.49 is close enough). Dropping to 10.3 when cranking doesn't sound so bad to me (that starter is pulling a lot of juice.)

You could check the FPR by replacing the return line with a piece of 6mm (1/4") vinyl hose to a suitable container. I get about 1/2 liter/min with the fuel pump running (engine off). The fuel return flow should only run for a second or two once the pump stops; it should drop the pressure to the bottom of the engine-off-spec and stop. If it keeps running until the pressure hits zero, you could clamp it off and try again. (The pressure can go to zero through a leaky check valve AND a good FPR, so be careful interpreting your results.)

The real question, though, is "why do you care?" What is the problem you're trying to diagnose?

Co_94_PU

iTrader: (-1)

.. Anecdotally

The voltage regulators in the ECU are good down t about 9.6 +/- a few diodes (~=0.3+Volts)

..
What scope says, what's the problem?

outdoorjunky

right I understand the check valve but my fuel pump is also brand new Denso, how often have you seen a bad brand new fuel pump for one of these? While trying to wrangle this problem I replaced both the FPR and Pump.

I guess one thing I need real education on is the flow path of the fuel lines ... my understanding is the pump pumps it in to the fuel filter and then the rail and excess fuel goes out the FPR to the soft return ... but does that soft return fuel line end up dumping back into the fuel pump? I thought that was a separate path back to the tank? I guess my thought process had me thinking that pinching the soft line was proving that the rail was holding pressure and that meant the check valve was too. Am I missing something on the signal flow here (I probably am)? the one thing that hasn't been replaced with a new part is the injectors (including the cold start). The Cold Start sprays with 12v from the battery but I didn't confirm leakage or not because at the time I was also having a problem with the thermo time switch circuit to the injector.

Basically it's a misfire that comes on when the engine idle drops as the engine is warming up... Cold it runs smooth but the second that idle drops the mis comes in on idle and really shows its head at around 2k rpm to about 3500 or so. The EGR vacuum modulator was suspect but I replaced that, engine runs better but not a cure for the misfire, the EGR valve tested ok the engine dies when vacuum is applied, and I currently have the EGR blocked off at the plenum with the aluminum can trick to make sure it wasn't leaking causing a lean condition but no change. Also the thermal BVSV has no vacuum when cold and passes vacuum when it's warm.

If I unplug the O2 sensor the mis stops completely for a while then will return after it runs for a while. If I jump the fuel pump and unplug the connector from the AFM the misfire is non existent at idle also ... of course idle is all I can test with the AFM unplugged. I unplugged the TPS to see if the engine would run better without it plugged in and the misfire really shows up at idle much more prominently than with the TPS plugged in. The TPS is new OEM and I calibrated it multiple times, the O2 sensor is Denso and also new. I've been through ignition stuff and tested compression everything is good or replaced if it wasn't. It's an annoying issue .... sometimes If I have the battery pulled and the ecu gets reset the engine will run without a mis for almost an entire test drive in a few cases an entire 30 miles run and then it's back the next day. I tried a second ECU known good from a running truck and no change. igniter wiring is redone and I ran a back up igniter and a new coil already as well. I'm reaching the point of running out of things to test or replace.

As far as battery I'm probably due for a new one soon.

outdoorjunky

I was typing up a reply to Scope while you replied but that reply to scope has the info about the problem I've been having for a while. thats most of it but there are some other parts that were tested, including AFM, ECU pins for voltage, resistance, and ground to body. I ran a second AFM because I got a few 0.L readings in the sweep test so I thought that could be causing the misfire by messing up the air flow readings to the ECU but the second AFM didn't change the misfire. .. I've tested the wiring to the TPS, the AFM, the water temp sensor (green sensor on front of lower plenum) I also did a resistance test of the water temp sensor in room temp water to boiling and watched the resistance readings compared to the FSM chart and it was all within spec.

outdoorjunky

Since this thread developed into one about fuel pressure more so than the efi relay I wanted to stop by with the solution to the fuel pressure problem ... it was a leaky cold start injector. I sent a backup set of injectors including a cold start injector to witchhunter for “rebuild” and when they came back The data indicated the cold start was sticking. I installed it and now the fuel pressure holds once the engine is off. It used to bleed off immediately but now it drops to 22psi and holds for about 5-10 minutes before dropping off. It’s within spec now.

87topless

When testing the EFI relay socket on the fuse block, what connections did you test? I pulled my relay and testing the socket on the fuse panel, I can get 12.5 volts on the EFI terminal (upper right if looking at the fuse block) and can get continuity to ground on the opposite terminal (lower left). But no continuity or power on the other two pins. I have tried with the key on, key off, and cranking the engine. The relay passes the continuity test outlined in the FSM. I thought the upper left socket would have power if the key is on or cranking the engine.