I tried a search, but I had no luck. I know what fuel pressure should be at pump and when tied and engine off with primer jumped, but what about while cranking under normal conditions?


I see 20 psi while the engine is cranking, is this normal?

Plus, 6 ohms at the injector solenoid is ok correct?

I have a huge flooding problem, I am trying to isolate a cold start issue I have posted in another thread.



Thanks for the help.

 

20psi is too low.
It should be more like 30psi.

If you are having a FLOODING problem, something is really screwed up @ 20psi.


-Ted

 

If I should have 30 psi when cranking and 37 psi when runing..what can I check/test that could cause the low pressure during cranking?

 

It should have minimum 30psi.
According to the FSM, it should actually spec out to 40psi?

There is an FPR solenoid that clicks open, and the FPR should see "0psi", so you're supposed to be seeing 40psi at the rails.

20psi is just too low...


-Ted

 

Could a weak battery cause that?

 

Well my pressure is fine if I jump the pump. . I have a areomotive FPR set to 36 psi when I jumped the yellow clip, but when I try starting it (with jumper properly removed) I only see about 20 psi .

I thought the PFR solenoid only plays a role when the engine is running and vacuum is present, I'm confused now...sigh.

 

You're running an aftermarket FPR?
Shoot, I thought it was all stock...sorry.


-Ted

 

Just reinforcing the above. The pressure should be 37 to 40psi during the start. Mostly because there's no vacuum to pull a STOCK FPR down to the 28-30 psi range.

So? Why not start with the jumper installed so you'll have regular pressure???

And if there's a open in the water thermo sensors wires to the ECU, then the injected amount will be toooooo litttttlllllle and cause for a very hard start during cold starting conditions. An open wire will not effect a hot engine start though due to the ECU defaulting to 176 F if the wires are off the water thermo sensor or there is an open to the ECU.

 

Well I know for sure my thermo sensor is good. I ger about 2.5 volts at ECU and get 4v when I unplug it, so thats fine. I am going to look for fuel resistor solenoid specs since I cant find it in the manual and go from there. Hopefully I am heading in the correct direction. I might also double check the AFM again...who knows? Some thing just aint right.

 

Stock, early 87/86 are six ohms. See the attached jpg of one.

 

The FPR solenoid is only activated for hot starts if the ECU sees coolant temp over 140degF and intake temp over 149degF, and only has any effect after starting, because like Hailers said there's not enough vacuum during cranking to bring fuel pressure down. During cranking it should basically see static fuel pressure, i.e. 36-37psi.

Measure the fuel pump voltage while cranking. It sounds like there's an issue with the pump voltage dropping during cranking.

Why do you have an aftermarket FRP anyway? Your mod list doesn't seem to justify one. You don't mention an upgraded pump...

 

As far as voltage, I still have to check, but I suspect the pump is getting voltage through the pump resistor rather than a full 12v as is should when cranking. I am going to pull the pin at the ECU that grounds the resistor relay and see if I get proper pressure during start.

As for why I replaced the FPR, in the past I suspected the stock FPR was bad and I had plans for a after market pump and higher boost. Now all I want to do is get it running and sold. I have had it with rotaries..clearly they have it out for me as this is my 2nd 7 with no luck, my other a 1st gen with a 13bt that failed.

I am determined to get it running though..I refuse to sell it without taking it for spin after all my hard work!

 

Check the voltage first, and if it's low during cranking then check the relay behind the right headlight. It's very unlikely the ECU is the problem so messing with it is the last thing you do after checking everything else.

See the FSM pages 4B-72 and 4B-73.

 

Can someone link to a basic write up for electronically retards folks like me.... im trying to fix my cold start issue and think i have stumbled across a good answer in the above posts.... What id like to know is how to either check if there is an "open " in the water thermo sensor wires....

then from there... if it does'nt solve the problem, how to check to PSI of the fuel supply at the rail and wherever else makes sense.... Thanks for any help...Also, i have no upgraded fuel system components... it is a 88 NA with a complete rats nest removal.

 

Here's another question (semi-hijack). I've got an aeromotive FPR with 720/1600 and Supra TT pump. I was planning on setting the fuel pressure by jumpering the fuel pump check connector and turning the adjuster to about 40psi. Is that what it should be at without the car pulling vac? Or is it 37psi? Don't most piston engines run at 43psi?

 

The easiest way is to back probe with a meter, the green/white wire at the ECU with the plug connected to the ECU. Key ON, Meter neg lead on a known gnd. With a hot engine the reading should be a half volt i.e. 0.4 to 0.5vdc. If the engine is cold, the reading might be as high as three volts plus. Monitor that reading with the engine warming up and the reading should decrease continually til the car is hot and the reading is a half volt.

If you INSIST on ohming the thing out, pull the middle plug OFF and read the green/white wire to ground. Meter on ohms. Key OFF. The reading should come close to matching the figures in the FUEL section of the FSM.

I assume this is a series four car.

If you had a RTEK2.0 you could simply look at the displayed water temperature on the Palm and see the coolant temp. No coolan temp........disconnected sensor.

 

Tee a pressure gauge into the line between the fuel filter and the fuel rail. Jumper the fuel pump check connector, turn the ignition on and read the gauge. Should be 36-37psi.

FC's run 36-37psi. Don't care much about other cars. I assume with a fuel system like that you're not using the stock ECU, so set it at whatever you want and tune accordingly. 40-45psi is fine. If you have a stock ECU you need to run stock fuel pressure or use a fuel controller, otherwise increasing the static fuel pressure will cause constantly rich mixtures.

 

yes, series four... what if my readings arent near those.. or i get no reading... would i assume that the sensor is bad or that there is a break somewhere in the wires between the ECU and the sensor?

can i test the sensor itself?

this is the sensor threads into the rear or the t-stat housing? green plug?

 

Update... followed the instructions... ran cold at 3v and as it warmed up it slowly dropped... then settled at .45v... sounds good?

So... if my problem is really hard cold starts.. sometimes needing to spray a bit of starter fluid.... where else should i start looking?

 

Yeah. That's a normal water thermo sensor. BAC plug connected? It holds to the open positon during START. Adds more air.

Welllllll, there is another possibility. Put that meters positive lead in the backside of pin 3B, a black/blue wire on the ecus' small plug. Bottom row, far right. Connector connected up to the ECU. Put the neg lead of the meter on a known gnd. Like a ECU mounting bolt/stud.

Go to START and see if the meter reads b/t 9 and 12vdc when the key is held to start. Let go of the key and there will be no voltage. IF you did NOT see 12vdc as the key was HELD to START, then that might be the problem. Check that out.

The ECU is looking for that good water thermo sensor signal that you know is good. It's also looking for the Start signal when the key is held to Start. And it's looking for rpm's under 500rpm. No 12vdc on the 3B wire means THAT is the problem. IF that were the case, then the ECU will use the afm signal for fuel amount instead of the MAP inside the ECU that is normally used for Starting fuel.