Fuel system diagnosis

[gsbarry]

Hey y'all,

I'm looking for some opinions on what parts I should plan to replace on the fuel system (i.e. both LH and RH fuel assemblies, one or the other, or any serviceable components). What I'm observing is that when the fuel gauge reads near empty, I'm only able to fill ~9 gallons until no more fuel can be added. When adding 9 gallons the fuel gauge reading goes from near empty to completely full. So it appears that the RH fuel tank either is full and won't discharge fuel into the LH tank, or will not fill. I am not observing any DTC's, and the fuel gauge otherwise functions normally (i.e. not erratically), other than being effectively twice as sensitive to the amount of fuel filled/consumed as it otherwise would be. I'm also not observing any starting or drivability concerns.

I brushed up on the function of the fuel system here. One part that caught my eye, "Zone 4 -- The LH voltage is below full threshold value. Fuel volume = volume in LH tank." which describes the function of the fuel gauge. Which to me reads as the fuel gauge will basically ignore the RH tank fuel sending unit as long the LH tank is not completely full, which seems to align with the behavior I'm observing.

I also noticed this section "Zone 5 -- The LH voltage is below its full threshold and the RH voltage is above its empty threshold. Fuel volume = volume in LH tank = volume in RH tank. This is a condition that should not occur in normal operation, because the RH tank should be consumed before the float in the LH tank begins to drop. If the fuel system is in zone 5 for a certain amount of time, a DTC (1431, 2066 or 2636) will set, and the fuel volume will be reported as zero by the fuel gauge." Since I'm not getting a DTC or empty level/non-functioning fuel gauge, it suggests the right hand fuel sending unit is reading empty (i.e. not above empty), which means the right hand tank is either actually empty, or perhaps the right hand sending unit has failed and is giving a false empty reading.

Lastly, this caught my eye as well, "If the secondary regulator (right tank) is not seated, it could result in fuel not transferring from the right tank." So it could be possible the RH regulator is not seated and the RH sending unit has failed.

With all that said, and considering the difficulty of removing the tanks (which I plan to do myself), and the cost of parts, which parts specifically do y'all think I should plan to replace (i.e. both LH and RH fuel assemblies, one or the other, or any serviceable components)? And can you recommend any parts sites with reasonable prices, and/or after-market brands of good quality, if any? I'm generally not in favor of the "parts cannon" approach by just replacing everything, although sometimes that's justified based on installation difficulty. If there's a procedure to bench test any of the components upon removal that would make sense too. Although the issue seems to be with the RH tank, I'm not certain the part failure actually lies within the RH tank assembly (i.e. I'm speculating it could also be a faulty check valve in the LH tank assembly causing the RH tank to malfunction).

Thanks for reading! GB

 

[xlrlist01]

Do you have a Tech-2?

It shouldn't be possible for the RH tank to fail to fill, there is nothing clever about it, gas from the LH tank over-tops the connecting hose and it pours in. It is normal for the LH tank to be full and the RH tank empty, if you don't put enough gas in to fill the (empty) RH tank. Ie, as I have interpreted how the filling system works.

Lets suppose the gauge (correctly) shows 1/4 full. In this condition the RH tank is empty and the LH tank is at 1/2. You now fill with gas. The LH tank will fill to full. Now continuing to fill with gas, the excess pours into the RH tank. The pump system will empty the RH tank fully before starting on emptying the LH tank. The gauge display system knows how to show the correct levels.

I'm gathering from the above you already know all this.

There was a recall on the gas tank system (leaks).

Can you tap the two tanks and 'sense' the level of the gas from the sound?

It seems surprising to me to have two failures at once. Not impossible. If you do have a full RH tank, you will have to handle it carefully, it will be very heavy. Is it possible something has somehow blocked the connecting hose? Could somebody have forced a plastic bag down the filler?

When I have read up about changing the gas tanks, it is a major job, do you have a full workshop manual? Similar year Vettes use the same system.

 

[gsbarry]

Thanks for the reply.

A Tech-2 would be quite useful in this situation to validate the RH sending unit voltage level. Unfortunately that's one big purchase I've avoided all these years. The vehicle has been garage kept and rarely parked on the street so I think it's unlikely someone deliberately clogged the fuel system, but you never know. It does seem more likely to me as well that based on the design of the system RH tank is currently full and not discharging into the LH tank. That just doesn't seem to jive with the lack of a DTC, unless the RH sending unit has failed as well (perhaps even the float is stuck in the empty position - mechanical failure), who knows? I'll see if I can get under there and sense the level audibly, good idea. I would think an electrical failure of the RH sending unit would trigger a DTC, if anyone can confirm this.

I haven't observed any fuel leaks or odor, so at least we can say that's not the issue.

I have a pdf copy of the factory service manual, but for some reason the fuel system section is missing. If anyone has an electronic copy I could look at, pls PM me, I'd be very grateful. I am aware that the manual calls for removing the exhaust, rear suspension, transaxle, but that it is possible with some skill to avoid all this and just get the tanks out directly.

GB

 

[mickeytee]

You just need to drop the rear crissmember about an inch to get the exhaust out. If doing on the ground it needs to be fairly high in the rear to tilt the bend of thevexhaust out. Put jack stands inder the rear frame then jusck up on the rear crossmember. Loosen the 4 crossmember bolts.all thevway then turn them about two turns back into the frame . that will give you room to remove the exhaust. The twisting of the clamp on the crossover is tough. Replace the transmission cable rear bushing while you are working on it. The tanks are fairly eay to remove. The hard part is fishing the pump and lines bank into the tank. Ive done a few. Its work but doable. Dealer cost is about 2500$

 

[gsbarry]

Thanks for the tips Mickey! I have a few follow-up questions for you if you don't mind:

1. Do you typically replace both LH and RH fuel assemblies when you do this job? Or is there a diagnostic procedure you follow to isolate one side or the other? Or even subcomponents?
2. I've watched a few videos of corvette owners doing this job without removing the exhaust and/or lowering the cross member. They do seem to struggle and often break the crossover tube though. Do you do this because you find it much easier overall to create the working space/visibility?
3. Before you lower the crossmember from the frame, are you removing/loosening any other parts/components? Or given that it's only moving one inch, it's not necessary to touch any other parts before this?
4. Roughly how high are we talking to jack up the vehicle for the exhaust clearance (I don't have a lift)? Do you think jackstands alone at their upper limit is high enough? If needed, I could get more height by putting jackstands on top of stacked wood blocks.

Thanks again!
GB

 

[mickeytee]

1. It takes a tech two to determine which side is bad. The ones ive replaced were due to a crack in the pressure line so i usualky do both. However the pumos have shot up in price for gm and ive not tried an aftermarket. Once i find a shop that has done aftermajet for a while, i nay go with them and then yes i woukd do both.
2. I think the corvette ones are doing them on nanual transmissions which gives you more room. The automatic is tight.
3. The exhaust is so simple to move fir me i just do jt. I remove the x pipe also . a little rust buster on the bolts and the slip joint and a littlw twist ,the mufflers come out pretty easy. I also invested in a kevlar arm sleeve . you are reaching up and around a few sharp areas. Other than the transmission cable thats it. I do it like i said to replace it.
4. Hard to offer advise on the jack stands. If im on the ground i jack the front and sit it on the drive on ramps. Then i jack the rear crossmember as high as it goes with the jack. Put the satnds under the frame about the door edge area . tthen once the crossmember is loose i can let the jack diwn a little but leave it under the car.

 

[gsbarry]

Thanks Mickey for the additionnal info. Based on XLRlist's comment about the RH tank not being able to fail to fill, I was doubting myself and decided to double check my observations last night. In the last few years I haven't been putting many miles on the vehicle, so to avoid fuel getting too old/stale for my limited consumption, I've typically only put ~5 gallons in at a time. A couple of weeks ago I put in ~5 gallons in the tank (and little driving since then), and was able to add ~11 gallons last night adding up to at least ~16 gallons. It's possible there was ~2 gallons in the tank even though it was reading right at empty on the fuel gauge prior to the last ~5 gallon fill, which would get back to the total ~18 gallon capacity. So it seems I am actually able to fill both tanks, and this also suggests the RH tank is properly discharging into the LH tank (I'm inferring because there was enough empty fuel tank capacity to begin with). I also confirmed with a basic scan tool that there is no fuel-related DTC. With that said, I'm thinking what I have is just a fuel level reading problem, not a fill/discharge problem.

I do recall with certainty that the fuel gauge went from empty to ~half full with only ~5 gallons being added a couple weeks ago. Based on what I've read about the fuel "zones" this leads me to believe I have a faulty RH fuel level sending unit (either electrical or mechanical failure possible) that is falsely reading empty on the RH tank.

I think one more possible confirmation step would be to record my mileage and see if I actually get ~300 miles out of this latest fuel fill up. If so, this would all point me to only replacing the RH fuel sending unit, if I didn't want to replace everything else preventatively. During this latest fill, and once more some time ago, i added a bottle of Techron as well, just to see if this resolves my issues, but I'm not holding my breath on that, especially given this is likely a sending unit issue and not a pump issue, which I wouldn't expect a cleaner to correct.

Has anyone tried out the Chinese clone Tech 2's like this one? At ~$270 it seems like it might be worth rolling the dice and if it works then great. But if y'all have feedback that they don't provide the functionality needed for the XLR's then I won't waste my money.

Thanks,
GB

 

[mickeytee]

I think most of the owners that use one have a clone. Rarely does a genuine one come up for sale for less than 500. I know a couple with clones that are very happy with them

 

[gsbarry]

Well I decided to order the Chinese Tech 2 clone. Fingers crossed... and we'll see what it reveals with the RH sending unit.

 

[mickeytee]

It will only take one sucessful relearn on the top at some point to pay for the unit. The fuel system is a little harder to diagnose but the info is out there

 

[gsbarry]

Does this look like the screen (taken from a youtube video for another vehicle) I should navigate to once my Tech 2 arrives in order to get an idea of what's going on with the fuel system? Or is there other diagnostic info I should navigate to?

 

[justdennis]

That looks right but there are many screens with some of the same information, on our cars there will be two level sensors, left and right. On my 07 XLR V when a tank is full it will read about .72 volts and the volts will go up as the tank gets lower, empty is about 2.45 volts. The brain on board will take both readings and calculate the fuel level you see on the dash.

I am using a Tech2 clone that I purchased last year.

 

[RubyRod]

When I removed my mufflers I didn't mess with the crossmember. I did remove the rear sway bar. But I separated the exhaust pipes at the exhaust manifolds (at the engine) before trying to separate the mufflers/pipes at the rear. This made it much easier to separate the muffler pipes and twist the pipes over the rear end. Before separating the exhaust pipes at the manifolds remove the 2 bolts for the spring loaded hanger near the transmission. I also didn't remove the mufflers/pipes completely. I just let the chrome tips rest on a piece of carboard on the ground.

 

[mickeytee]

We with fat hands need more room. Haha

 

[gsbarry]

My clone Tech 2 arrived over the weekend and so far it seems to be functioning well. I found it pretty funny just how large this 25 year-old technology is... anyways, below is the reading I have on the fuel sending units. As you can see both seem to be reading correctly at nearly full fuel levels. I'll continue to to monitor my readings... maybe I was just hallucinating this issue... or maybe its intermittent... time will tell.

 

[justdennis]

For some reason my tank level voltages are just the opposite, full is the lower number. Everything I have read says it should be as yours are but it's working so "if it ain't broke don't fix it".
I think you will like the Tech2, there is a lot to learn and so much information but for a car guy it's big fun.

 

[gsbarry]

Maybe the wires are reversed on your sending units?

 

[justdennis]

I'm not sure but the gauge is accurate so I can live with it.