Renix fuel pressure

I did some poking around here, just wanted to verify before Ben Franklin and I go out to the driveway and fix it: '90 Limited, 4.0, auto tranny.

So I've been chasing down this hard start problem, sometimes I think it should run smoother than what I'm seeing. Yesterday I borrowed a gauge to check fuel pressure on the rail:

Key on-engine off I get 40 lbs right away, then pressure drops to 10 lbs in the first 30 seconds or so, 0 lbs after 60 to 90 seconds. We put a fuel line clamp on the supply line and pressure held.

Key on-engine running we had pressure in the low 30s, remove the vacuum to the regulator and get 40 lbs again.

We did not pull the injectors, could not do a pulse test without stable pressure.

At this point I'm thinking it's time to replace the fuel pump. Am I wrong in thinking the presure at the rail should hold for longer than a minute? Is there a check valve somewhere other than the pump it-self? I really don't have the extra coin to just throw money at it.

TIA, Mike

As far as I know, the fuel pump has a check valve that should hold pressure for 30 minutes or so - but probably not overnight. There are those here who have installed secondary check valves just ahead of the fuel pump with good results, a search should turn up threads on that (including sources!)

RENIX fuel pressure, as I recall, should be 39psig with the vacuum disconnected from the regulator (engine idling,) and ~31psig with the vacuum connected and the engine idling.

Loss of fuel pressure after engine shutdown is a common RENIX issue, and can be tolerated if you're not losing it too quickly (but it sounds as though you are.) I've noted that my RENIX rigs take longer to start first thing in the morning than they do when I'm running errands - it seems that I lose pressure if I let them sit for more than about two hours - but I've got bigger fish to fry, and I've since learned to live with it (and they're on a "restomod" schedule anyhow - I'll deal with it then!)

5-90

Key on-engine off I get 40 lbs right away

Click to expand...

Then the fuel pressure is not the problem.
Does it turn over really fast?

The FSM for my '88 4.0 says record pressure at idle, shut down engine, let set for 30 minutes and recheck pressure (engine still off). Pressure should be 19 to 39psi. Excessive drop indicates bad check valve. Your pressure readings are good, so it's probably the check valve, although you could have a leaking fuel line INSIDE the tank, allowing the fuel to leak back. The flow rate for the pump is 1 liter per minute.

The check valve is not great, but not so bad as to cause a slow start.
A check valve related slow start keeps your fuel pressure low or even 0# with the key on, fuel pressure builds up slowly while your cranking. You have instant 40# with the key on so it's not the fuel pump or check valve

hey there im trying to track down the same problem. i know it takes a little longer but i was having 4 full revolutions be for it would fire.

90xj06 said:

hey there im trying to track down the same problem. i know it takes a little longer but i was having 4 full revolutions be for it would fire.

Click to expand...

With a good starter 4 full turns would be less than 3 seconds.

well i have not counted but its a brand new starter and it is spinning fine. i think its the cps. but im going to check all the grounds i know there bad somewhere. i have high resistance from the engine ground to the battery negative post.

ok guys --- side question --- wheres a decent place to get the pressure guage for screwing on the fuel rail? thanks

I got mine at a local parts house without too much trouble, I think it's "K&D" branded. Don't go to a "national chain" shop (do you have any idea how many times a day I say that...?) for something like this - go to a mom&pop or a "local chain" (say, 10 stores or less) because they'll deal more with repair shops, and have a larger network of local WD's - if they don't have it, they can probably get it that day or the next.

The RENIX fuel rail uses a GM-type Schrader valve, I don't know about ChryCo/HO. Also note that some late models don't have a test port on the fuel rail - requring the use of a "T" fitting, per FSM. Check your application carefully before buying tools...

I seem to recall that I paid about $30 for my test gage.

5-90

langer1 said:

Then the fuel pressure is not the problem.
Does it turn over really fast?

Click to expand...

Yeah, it cranks fine. New starter and heavier than stock cables, new battery clamps. It just dawned on me that the test is kind of skewed, I had to drive to borrow the gauge. The problem is worst after sitting, especially overnight. You can crank seemingly forever, stop and cycle the key again it starts almost immediately.

I guess a little more background is in order. My landlord teaches engine performance at a Vo-Tech, we used the toys...uhh...equipment at his work. He has never seen pressure drop this quickly on FI, that is why we didn't proceed with the injector pulse test, or bother with a volume test. Since we had the injector harness disconnected for the pulse test, I watched the pressure while cranking. It did slowly come up to the 30 to 40 psi range.

We had a vacuum gauge hooked up, 15" manifold with a slight fluctuation. I think 19" is the best he said he has seen at this altitude, thought 15 was a little low. Goose the throttle, the manifold vacuum dropped to 0" immediately; no exhaust restrictions. We used a smoke machine to look for leaks in the manifold, that was slick. Looks like I'll be replacing the EGR valve soon. He didn't think that leak was big enough to cause a hard start condition, but the valve should seal all the same.

Past that I'm still cleaning electrical connections, and looking to replace a few I've broken already. Probably wouldn't hurt to pull the plugs, see how they look. I replaced the plugs, wires, cap & rotor, TPS in early July. Same time I did a dry compression test: 120 psi lowest, 135 highest, 1 @130 psi, 3 @125 psi, I didn't bother with a wet test.

Sorry about the novel, but if I'm going to ask what I've missed, I guess I should include what I've covered. Seems it's always the pesky little details that whoop my ass the worst. LOL

Now to see what the Jeep guys think...

Hmmm... Two more hours of searching...looks like I'll be inspecting the line inside the tank like xjbubba mentioned, then on to the BMW check valve, not to mention the filter. I'm tempted to drop the tank and pop the dents out, check the vents. The thing is a PITA to fill, tough to be consistant.

I'll post findings, Mike

thanks 5-90

It did slowly come up to the 30 to 40 psi range.

Click to expand...

The you do need to look at the pump and or filter, the injectors will not open below 30# or so.

On the subject of 3-4 revolutions to get the engine to start - IIRC Renix needs that to get the firing order timing off the CPS when it's old and weak.

It takes mine that long every morning, the pump howls randomly, and with the amount of blowby I have it's a wonder I can get any vacuum. Ain't it great?

TiRod said:

On the subject of 3-4 revolutions to get the engine to start - IIRC Renix needs that to get the firing order timing off the CPS when it's old and weak.

It takes mine that long every morning, the pump howls randomly, and with the amount of blowby I have it's a wonder I can get any vacuum. Ain't it great?

Click to expand...

The Renix needs 3-4 every time good or bad CPS.
The renix has no memory to store timing information so needs to reset it on every start.

The problem described is a bad check valve in the fuel pump, the pump inside the tank.

There are two ways to fix this:
1) change the fuel pump
2) install the BMW check valve and continue to use the pump.

To install the check valve you cut the larger rubber line between the tank and the fuel filter and install it using hose clamps, job done, starting problem fixed.

The perminate fix is to install a new fuel pump, I did this on my 88 XJ in 2000. I got the pump kit at the dealer and it cost around $200. The dealer kit is VERY nice, came with a Bosch fuel pump. It included the new wires and rubber pieces you need to use. The replacement pump had 1/4 inch spade terminals, the 88 factory pump had 1/8 inch terminals, that why the new wires are so nice.

If you change the fuel pump you need to pick up a O ring for the tank flange plus a sock filter for the tank. You change the fuel pump without dropping the tank but if you have use of a lift it much easier since the axle will be dropped down.

The BMW check value is the quick way to fix the problem and with a working fuel pump it makes a lot of sense.

Like some of the other posts I could not find the listed part # on the web. After thinking it over I decided to just replace the pump. The one I brought home didn't fit the Bosch bracket to my liking, so I've orderd the pump with sender. List price is less than the Bosch pump, maybe we'll get a better read from the fuel gauge.

The fuel tank had a HUGE dent in the bottom. Big enough to bend the slosh well. Tried punching it out with a wooden handle, decided WTF! It's the mom-mobile, I'm not wasting time or money installing something I'll have to worry about later: New tank on order.