Fuel pump question - no prime at key-on?

[jawn101]

So this is a weird one, at least to me. I know when you turn the key on (not to start, but just 'on') the fuel pump should run for a quick second to prime the system. Mine doesn't. Well, on very rare occasions it will - I know I've heard it run before but not in quite some time.

But, the car does start and run just fine. One caveat would be that if I've left it sitting for a few days, it stumbles a lot at RPM above idle, at least for a few seconds. After that it runs like a dream, and will continue to do so unless I leave it sitting for more than a few hours. Idle is smooth and even after the lambda warmup. To me, that indicates that there's a lack of pressure in the fuel system until the engine/pump have run for a few seconds, so this seems to be symptomatic of the pump not starting up and priming the system. Does that make sense?

If so, what should I check out to see why the pump doesn't prime? My battery voltage seems OK - reads just over 12 with the car off. Alternator working properly, system voltage reads about 13.8ish on the dash when the car is running. Battery light illuminates when the car is 'on' so the regulator is exciting...

[Soundkillr]

Could be your rpm relay. If you turn key and it does not prime, see if turning off and re-turning key will make it prime. If mine has sat for several weeks i have to turn key twice (without starting) to get mine to prime. You are right, it does need to prime and after sitting for a few weeks, will run like crap if not primed. I have been to lazy to replace my relay, but it is on my list of to do's.

[jawn101]

Could be your rpm relay. If you turn key and it doen not prime, see if turning off and re-turning key will make it prime. Id mine has sat for several weeks i have to turn key twice (without starting) to get mine to prime. You are right, it does need to prime and after sitting for a few weeks, will run like crap if not primed. I have been to lazy to replace my relay, but it is on my list of to do's.

And I was just going to edit and add this - my RPM relay is new, an updated one from Hervey. I haven't gone as hardcore as to get Dave's solid state one yet, but it is a new relay. What other symptoms would I see if it had gone bad? I don't see anything else wrong, if I'm honest.

And no, I've tried killing the engine and turning the key back on/off/on a bunch of times and nothing - no prime.

[Bitsyncmaster]

Some RPM relays do not prime. My original RPM relay would prime every time unless I had a battery charger on the battery.

My solid state one will always prime. With mine the software turns the pump on at power up (key on).

[jawn101]

Some RPM relays do not prime. My original RPM relay would prime every time unless I had a battery charger on the battery.

My solid state one will always prime. With mine the software turns the pump on at power up (key on).

I saw your post about that actually, which is why I added the bit about my battery voltage seeming good. I don't remember if it ever really primed with this new relay. I still have the original one in a box somewhere, maybe I should swap it back in and see what happens?

Having the pump prime seems like an absolutely necessary thing though, given that the car runs for crap without it :) Maybe I'll have to get a replacement sooner rather than later....

[jawn101]

I also just realized that I'm a total tool for starting a new thread. I found Dave's posts about the RPM relay on teh googlez and for some reason thought it was an old archived thread from .com. Oops!!

[jawn101]

Tool or no, I just swapped my old RPM relay back in, and I still get no prime before starting the engine. The old relay was black with a blue base and appears to be a Bosch - original? Not sure.

[jmettee]

My original (dated April 1981) always triggered the pump every other turn of the key. It was pretty reliable at doing this too. I made that my spare & ordered one from Hervey, this one is a green case & it does exactly the same thing.

I would love to know what makes some cars do this. I can understand modern electronics that count instances & can do different things depending on conditions, but AFAIK the DMC electrical system should do the same thing every time. Not sure what makes the RPM relay prime the pump like a toggle...every other time.

Also, lately my car gets driven every other week. I purposely turn the key twice before starting to get a pump prime so the car starts immediately, as opposed to cranking & stumbling along before fuel pressure builds.

[Bitsyncmaster]

The pump is controlled by an IC that once triggered will turn the pump output on for one second. Two things trigger that IC. One is a pulse on the ignition coil and the other is when it powers up (if you’re lucky). Now does your ignition ECU provide a pulse on power up? Does that pulse occur after the RPM relay is powered up? Does the reset on that IC hold long enough until power is up?

There is not a circuit in the OEM design to guarantee a prime. This type of operation we call a timing race in electronic design.

[jawn101]

Awesome. That's the kind of answer I was hoping for - something quantitative. That's great, thanks Dave.

[Chris 16409]

Just buy Dave's solid state RPM Relay. It will be the last one you ever have to buy.

[jawn101]

So I hear :)

[jawn101]

So I got and installed Dave's excellent new relay, and while it does exactly what it's meant to and keeps the system primed, I still have hesitation/stumbling just after I start the engine. It only happens if I go higher than maybe 1500 RPM - so I can back out of the driveway, get into 1st and get to the entrance of my neighborhood (a few hundred feet from the house), but then when I go to turn onto the main road and have to hit the gas... that's when I suddenly have no power or even worse, stall out. After I get it going and travel a little way down the road (maybe 1/8th mile) the problem vanishes and fuel delivery/power is fine for the rest of the day.

Never have any problem starting or re-starting if it stalls (rare, as I typically will let off the throttle before that happens), idle is smooth and even after warmup, don't seem to have any vacuum leaks and have all new silicone vacuum lines. New accumulator, new fuel lines, recently tuned up engine. I guess my next thing to look at is the pump/pickup hoses...

[Bitsyncmaster]

It could be just a mixture setting or a bad CPR (WUR). Have you checked the mixture via checking the FV duty cycle (dwell)?

[jawn101]

It could be just a mixture setting or a bad CPR (WUR). Have you checked the mixture via checking the FV duty cycle (dwell)?

No, I don't have a dwell meter. I guess I should get one. Is it easy to eliminate the WUR from the equation?

[Bitsyncmaster]

No, I don't have a dwell meter. I guess I should get one. Is it easy to eliminate the WUR from the equation?

You need a fuel pressure test set to check the WUR. You need a dwell meter to check the mixture. The dwell meter check is easy one.

[David T]

Check the vacuum hoses, the mechanical and vacuum advance and the vacuum plumbing around the Control Pressure Regulator. Can be a vacuum leak too or dirty injectors. The motor might need a tune-up, replacing spark plugs, filters, etc.
David Teitelbaum

[jawn101]

Check the vacuum hoses, the mechanical and vacuum advance and the vacuum plumbing around the Control Pressure Regulator. Can be a vacuum leak too or dirty injectors. The motor might need a tune-up, replacing spark plugs, filters, etc.
David Teitelbaum

Vacuum is all fine, all brand new silicone. Engine was recently tuned up, all new plugs, filters, wires, etc. I "cleaned" the injectors as best as one can without a pressure tester during the tuneup but it's possible (if not probable) that they are less than optimal. Mixture sounds like a reasonable assumption, but I have to find a meter :) All my DMMs lack dwell. One of my friends around here must have one, if not Amazon sure does.

[Bitsyncmaster]

If you buy a meter, get one that has 4 cylinder range. That is the range used to get the values in our service manuals.

[jawn101]

If you buy a meter, get one that has 4 cylinder range. That is the range used to get the values in our service manuals.

Thanks, I was actually just about to ask that question in another thread that mentioned how to use the meter. I wasn't sure if I was reading it right since I'm pretty sure we have 6 cylinders :)

So, an update on this issue. I just spent the whole morning going over every inch of the vacuum, fuel and coolant lines (I was seeing a tiny puddle under the car) as well as checking the spark plugs for fouling, and here's what I found:

- Vacuum lines - all good and solid connections. There's a tiny tiny leak at the removable plug where the tamper-proof cap used to be on the fuel distributor. I'm going to have to give a little thought to how to seal that, it looks like when the original/previous owner had that plug drilled out it formed a slight irregularity in the housing, which is allowing a very small amount of air in under the plug

- Coolant lines - found a leak where the passenger side pipe connects to the hose from the water pump. The leak was pretty small, but it was hard to trace because most of the lost coolant was going straight into the alternator, which accounts for some of the electrical issues I was seeing earlier. Sealed that up and cleaned/dried the alternator - no more coolant leak or electrical gremlins

- Fuel lines - this was the big 'a-ha' moment! A drip where the flex line from the distributor connected to the hard return line. I don't know how or when that managed to manifest as I replaced all the fuel hoses months ago and the entire system was tight as a drum at that time. Scary and really dangerous, I know. I checked every fitting again to be sure and nothing else was loose.

Went for a test drive and the difference is massive. More power at all revs, starting is fast and effortless. The true test will come tomorrow morning after the car's sat overnight - if it starts easily and doesn't stumble out of the gate I'll know I've finally nailed this one.

One downside though - when I was checking my plugs to see if they were clean (they were perfect), I snapped the plug in cylinder 1. I blame my "World's Cheapest" spark plug socket from Harbor Freight... the rubber seal inside was worn funny (after only having been used 1 time) and wasn't holding the plug straight.

Anyway, the ultimate outcome of that is that I have 5 Bosch Platinum 4s and 1 Champion copper plug. Really s#itty, so I will have to either go get another plug. Is there any major issue not to run the car like this? I don't need to do it for long, just a couple days while I find time to get a new plug :)

Thanks again to everyone for the help!

[Bitsyncmaster]

Many people get that leak at the return line to pipe connection. I know I did. Even after cleaning the threads as best I could, it still needed a sealer on the threads.

[jawn101]

Well, you can bet I'll be checking that fitting periodically. Especially as it hangs so close to so many electrical components, I feel sort of lucky not to have been blown up :)

[SIMid]

Just on a side note, where can I get the Solid State RPR Relay?

Mine primes most of the time, occasionally she doesn't unless I re-turn the key again.

[jawn101]

Just on a side note, where can I get the Solid State RPR Relay?

Mine primes most of the time, occasionally she doesn't unless I re-turn the key again.

Talk to Dave (Bitsyncmaster) above. He designed, manufactures and sells them.... Click the link in his signature for more info.