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-   -   B21FT Impulse Relay Function (https://forums.turbobricks.com/showthread.php?t=300158)

smoothdurban 09-08-2014 02:42 PM

B21FT Impulse Relay Function

I have a 83 240 turbo with the original b21ft in it and I was wondering how the impulse relay works during warm starts.

Referencing the green book it says "during warm starts the impulse relay is engage after approximately 1.5s. It then gives a 0.1s pulse of injection with a 0.3 second interval"

My question is how long does it perform this pulse on / off cycle?

The reason why I am asking is during operation in warmer weather (80F+) I noticed that after starting the car stumbles for a few seconds (like 5 - 10 sec) at idle then it runs perfect. Normally this occurs for a relatively short period of time but last Friday we had a very warm day 90F+ and this bogged idling occurred for even longer almost like 30 sec. This condition only occurs after the car has been sitting for about an hour and a half after I have been driving it and does not occur when my car has been sitting in the parking lot all day at work.

My thought was that the thermal time switch is telling the the cold start injector to fire when it doesn't need to but after reading the description of the impulse relay I think it is related to it.

Any help would be appreciated.

keesue 09-08-2014 04:37 PM

It keeps on pulsing until either the engine fires or you let up off the key. It isn't driven by the thermal time switch. It takes power directly from the starter solenoid and once it reaches it's delayed trigger time, it will pulse. If your thermal time switch is faulty, it will tell the cold start injector to work when it shouldn't. The thermal time switch completes the ground to the injector based on engine temp. You can test it with a light. Disconnect the cold start valve harness and connect to the two pins. Disconnect the coil wire and crank it and time it. You'll se the light come on based on engine temp - longer when colder and a quick flash when moderately warm. Stay on the key when the light goes out and the impulse relay will kick in.

Rich running when starting warm is more indicative of a faulty vacuum temperature valve or the vacuum delay capsule in the control regulator vacuum hose circuit. If that little capsule on the regulator or the one in the head is bad, it will richen the mixture when warm. The way to test it is to put a vacuum gauge in the hose coming from the regulator on the head to the control regulator to see if there is vacuum when the engine is warm. Then, reconnect to the vacuum capsule on the regulator and see it it allows vacuum when warm. Either condition is bad for the regulator and will make the engine run rich until the valve on the head closes. When they fail they fail upwards temperature-wise.

Hope that helps.

smoothdurban 09-08-2014 04:57 PM

Are you talking about the thermowax / thermostat valve on the head and the, color side away from CPR, delay valve? Those are both brand new as well as the CPR since May of this year.

The idle bog I get stops in an instant and the engine idles normally. I will try to quickly disconnect the cold start injector when the engine is doing this and see if it has any immediate effect.

keesue 09-08-2014 06:16 PM

Ah, new is good. Didn't realize that or I would have held my tongue. :-P. I just went through all of this and I had this problem. Sorry 'bout that...

smoothdurban 09-08-2014 06:20 PM

no worries I always appreciate good input. I tried starting the car a few minutes ago after sitting for a couple of hours and sure enough it started with a rough idle. I quickly tried to discoonnect the cold start valve and I there was no change in performance. The after a few seconds it went back to normal.

Very weird. I will try probing the cold start valve connector for my next and see if it is pulsed at all.

dl242gt 09-08-2014 07:49 PM

Take a look at the fuel pressure accumulator. If it looks old and rusty on the outside. The inside looks like that as well. It dampens fuel pulsations from the pump and assists with a fuel pressure reserve for hot starting. It is sadly an expensive part nowadays. It's also one that is easily overlooked with hot start issues.

smoothdurban 09-08-2014 08:56 PM

Good advice. Its old but doesn't look that rusty. I know I had to remove the original tank due to rust so I am pretty sure all of that crud got pulled through the accumulator. My plan was to replace it along with a brand new fuel pump and that coupler that joins the fuel pump to the accumulator. I noticed IPD sells the coupler http://www.ipdusa.com/products/5026/...l-line-coupler but I don't know if this will fit my fuel pump/accumulator set up as it is for my 91 240 which I has the fuel filter next to the fuel pump.

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