Downsizing from 3L9 to 3L5, Who can help ?

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greenislander

New Member
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15
Location
Dominica (west indies)
Hi guys

Beg your pardon in advance for my so french english

I'm the happy owner of a 3L9 1993. I broke my old engine and di'nt find a 3L9 at a reasonnable price, and a 3L5 EFI from 1987 came out for next to nothing.

The 3L5 is in place and work, but not so well. I'm just a passionate and I'm not really skilled in diagnostic and i need some help to find the next check to do.

I wired the 3L5 with the wiring of the 3L9, and keeping my original electronic box. For the components on the 3L5 wich are not any more on the 3L9, i just remove them. Guessing that this 3L5 injection was the first one from RR, and was updated with few change for good reason. So i updated the 3L5 like a 3L9.

QUESTION : there is two water temp sensor at the front of the engine. Is the water temp the front one on or the back one. And what is exactly doing the other. My workshop manual is a little bit foggy on that point.

So
Compression are ok 140/145 PSI on the 8 cylinders
My sparkplugs are black, some are oily
When the sparkplug start to be dirty (it happen very quickly), i have blast in the exhaust pipe. The ECU is the one form the 3L9. I'm not completly sure of the timing of the engine.

QUESTION : The bad combustion could it be due to too much advance in ignition ?

QUESTION : How could i know (without opening the engine) if the oil is coming from the valve seal, or from leaking rings ?

Thanks by advance for reading or answering

Sunny regards

Greenislander
 

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sounds like it is running too rich a mixture and I am guessing your engine runs catalytic converters?
 
an '87 3.5EFI will be flapper, your 3.9 would have been hot wire EFI there are some differences between the two. Some componenets are not the same and setting them up is slightly different. For example, the air flow meters are different, the throttle pot settings are different and cold start control is different. i suspect you need to check where the air flow meter is set but also check values for throttle pot as you may be running rich. The coolant thermistor might be playing up as well.

I can only assume the 3.5 has no cats and your 3.9 was pre-cat as well. If not check what tune select resistor you are running.

As for your oil question, do a compression test and compare wet and dry readings. If it goes up when wet then possibly rings, if no change then valve stem seals.
 
Last edited:
Hi Kev

Thanks for your answer. Sorry, my english is still pour, so if your can comment the following points :

Except for the coolant thermistor, i let all the component from the 3L9. Step by step motor for iddle, gas thermistor, air flow meter, throttle pot. The only real difference is now 50CC per cylinder.


Flapper means "less power" ?

What are you calling no cats and pre cats?

Idem for "dry" reading and "wet" reading for compression test. I don't understand the difference. The mesure we did was done engine hot, all plug removed. Was it a "wet" or "dry" test ? The reading gave the same value 140/145 PSI for each cylinder.

The control of the coolant thermistor and gas thermistor are the next step. So the coolant thermistor is the front one or the back one (one of them is cold starter).

For the pot, i think i understand. When i throttle, the pot is sending to the calculator the position of the butterfly, so the calculator can adjust the quantity of gasoline injected to match with the butterfly opening. Is that that. So if i'm going one step forward, the pot could be cheated a little bit like some kind of adjustment screw. (Clock wise = more gas or less gas injected)?

And last, is it possible to affect combustion if the advance is a few degrees too early in the timing.

For cold starter, the engine is never really cold, i'm living in a tropical island.

Anyway, a lot of thanks to you kev for the time you took to answer me.

I send you a little bit of sun from the carribean.

Greatings

Gilles
 
Hi Fanatic

Thanks for answering.

No, the 3L5 like the 3L9 where'nt catalysed. I have'nt seen any lambda on the exhaust.

Could be the "too rich" be a consequence of the 50cc per cylinder between the 3L5 and the 3L9.

The saler of the engine told me that the canshaft are probably from a 3L9.

I'm swimming a little bit in all that (but the water is warm), and i still not drowned (LOL)

Sunny regards

Gilles
 
there is no cold start injector on the 3.9 injections sytem, is that they injections sytem you are using on you 3.5 motor?
 
in which case you should be able to change tune resistor (and or adjust mixture)
 
Thanks for your anwers guys

Fanatic, i saw on the book that there is a resistor, now i understand what for... Where do i find it ? Is it the tuning for mixture (up or down the value to reduce mixture richness) ?

Fett, i took the 3L9 out, and install the 3L5 with the components of the 3L9. Electronic box, Butterfly and pot, air flow meter, gasoline temperature, idle step by step. I only kept the coolant temperature from the 3L5. I wanted to avoid to change the wiring and everything, after all, the 3L9 is just an updated version of the 3L5 with 50 CC more per cylinder. Out of the fact that the 3L5 should burn a little bit less (probably a very small difference), the sensor should not see the difference, if they are working properly. Which is the question of the day. Maybe there was one wrong on the 3L9.

Thanks a lot to contribute to my culture. I'm in a small caribbean island, wher you have to be mac gyver himself if you want to survive, so i do...

Sunny greatings
 
you will need the coolant thermistor for the ecu to run properly as its in "limp home" mode without it and very rich like you said.

so you will need the 3.9 thermistor in the 3.5 engine you have fitted.
 
Thanks a lot fett, i start just to understand how it's working really. Tomorrow, I start to check the resistance by step of 10°C to see if the values are good for each temperature.

I did'nt precise, the injector are the one originaly on the 3L5, but they are just following the order of the electronic, are they?

Depending of the workshop manual, the resistance look to be the same on the thermistor of the 3L5 and the one of the 3L9.

On this old first generation 3L5, there is two "look like thermistor", one for timing cold start, and the other for collant temperature. So i guess one is proportionnal and the other is 0 under a value and 1 over. Basically, engine cold or hot. Is my guessing true ?

Fanatic is talking about tune resistor. Is that the way to adjust the mixture ?

I was also thinking to use the throttle pot to cheat. Is that realistic ?

Thanks thousand time for answering me, i feel a little bit a dumbass, but like y granny said, the only bad questions are the one you don't ask.

Because of you, my training in V8 RR Knowledge is going ahead very well.

Sunny greatings
 
no , the ecu is under the seat and is the main computer for the engine. the airflow meter is the hotwire unit attached to the air inlet
 
greenislander said:
Thanks a lot fett, i start just to understand how it's working really. Tomorrow, I start to check the resistance by step of 10°C to see if the values are good for each temperature.

I did'nt precise, the injector are the one originaly on the 3L5, but they are just following the order of the electronic, are they?

Depending of the workshop manual, the resistance look to be the same on the thermistor of the 3L5 and the one of the 3L9.

On this old first generation 3L5, there is two "look like thermistor", one for timing cold start, and the other for collant temperature. So i guess one is proportionnal and the other is 0 under a value and 1 over. Basically, engine cold or hot. Is my guessing true ?

Fanatic is talking about tune resistor. Is that the way to adjust the mixture ?

I was also thinking to use the throttle pot to cheat. Is that realistic ?

Thanks thousand time for answering me, i feel a little bit a dumbass, but like y granny said, the only bad questions are the one you don't ask.

Because of you, my training in V8 RR Knowledge is going ahead very well.

Sunny greatings
Click to expand...

you really want the block and heads formt the 3.5 and everything else fromt the 3.9 if you are trying to fit it this way as you only need the 1 thermistor as the cold start system is run from the airflow meter on the 3.9 normally.

the tune select resistor is needs to be changed if you had cats before but dont have them fitted now.

I think really if you are going to do this , like I say it just need to bethe block and heads form the 3.5 and all else like plenum, sensors,wiring form the 3.9-ie whats already in the car.

I dont know if or how well it will work, you may be the first to do this downgrade :D
 
Thanks a lot guys.

Your answer are driving me to several new questions.

I check your URL Mr noisy, I give me the answer about the tuning resistance, wich has nothing to see with mixture (thanks a lot Fanatic). A little chinese proverb for you "closed mouth don't gulp down fly".

Fett, that's exacly what i did, keep all wiring, ECU air flow meter and so from my previous 3L9 engine. I just kept the injector from the 3L5.
For now, until i makeshift an adaptation, i have just the thermistor for gasoline temperature plugged, but "in the air". The value for that one are fine.

How could i check if there is no troubble on the air flow meter ?

Does that gas thermistor influence a lot or little the mixture ?

In fact, it work, but bad. Still too rich. The spark plug are black like hell within 20 kilometers. I believe more in mixture troubble than ignition because when i swith to LPG, the running is a lot more smooth.

But at this point, it's more logical though than knowledge.

I check the coolant thermistor, and the value was "in the wind", 1190 ohms at 80°C while it should be between 300 and 400. So i get my old one from the 3L9, which is not so precse, 530 ohms at 80°C, but closer than the one from the 3L5. I was thinking to by pass it with a static resistor 350 ohms to see if something change.

I tune also the advance to 3° at iddle. It was roughly tuned at 9° before. It's better, but still too rich.

What should be advance at iddle. The manual say between 1 and 4,5. Closer from 1 or closer from 4,5°. I'm in a tropical country, and my engine is twin carburation, gas and LPG, with a vapo system.
Any ideas ?

So no, in waiting for a brand new thermistor, i would like to know if i can use the throttle pot to down the mixture. Theoricaly, it should work, but i prefer to collect info before playing aprentice wizard.

There is a little leak on my exhaust line, could it influence combustion ?

Thanks a lot for your help guys. Hope i will be useful for you one day.

Sunny greatings

Greenislander
 
greenislander said:
Thanks a lot guys.

Your answer are driving me to several new questions.

I check your URL Mr noisy, I give me the answer about the tuning resistance, wich has nothing to see with mixture (thanks a lot Fanatic). A little chinese proverb for you "closed mouth don't gulp down fly".

Fett, that's exacly what i did, keep all wiring, ECU air flow meter and so from my previous 3L9 engine. I just kept the injector from the 3L5.
For now, until i makeshift an adaptation, i have just the thermistor for gasoline temperature plugged, but "in the air". The value for that one are fine.

How could i check if there is no troubble on the air flow meter ?

Does that gas thermistor influence a lot or little the mixture ?

In fact, it work, but bad. Still too rich. The spark plug are black like hell within 20 kilometers. I believe more in mixture troubble than ignition because when i swith to LPG, the running is a lot more smooth.

But at this point, it's more logical though than knowledge.

I check the coolant thermistor, and the value was "in the wind", 1190 ohms at 80°C while it should be between 300 and 400. So i get my old one from the 3L9, which is not so precse, 530 ohms at 80°C, but closer than the one from the 3L5. I was thinking to by pass it with a static resistor 350 ohms to see if something change.

I tune also the advance to 3° at iddle. It was roughly tuned at 9° before. It's better, but still too rich.

What should be advance at iddle. The manual say between 1 and 4,5. Closer from 1 or closer from 4,5°. I'm in a tropical country, and my engine is twin carburation, gas and LPG, with a vapo system.
Any ideas ?

So no, in waiting for a brand new thermistor, i would like to know if i can use the throttle pot to down the mixture. Theoricaly, it should work, but i prefer to collect info before playing aprentice wizard.

There is a little leak on my exhaust line, could it influence combustion ?

Thanks a lot for your help guys. Hope i will be useful for you one day.

Sunny greatings

Greenislander
Click to expand...

Hold up, slow down!!!

Fuel thermistor tells the ecu if fuel enrichment is needed (it's like the choke!).

The coolant thermistor alters the length of the injection pulse, normally when these die it causes the engine to run rich. So yes, this is important in relation to the fuelling.

To check the air flow meter you need a decent multi-meter to check voltages and resistance.

The ignition timing doesn't sound right, but if you are running LPG you shouldn't use the standard settings, I can't remember if it needs to be advanced or retarded but I'm sure some kind soul who knows will tell you about that. In the alternative, you could try "setting it by ear" a bit of trial and error but you'll know when your engine is happy. When setting ignition timing make sure the engine is warm and the vac advance is disconnected and the pipe temporarily blocked, check the idle speed is not too high and make sure you mark your dizzy BEFORE you make any changes!

Slight leak onthe exhaust is not likely to cause too much of a problem.

If you do a search you'll find all the info you need right here on LZ.
 
Thanks a lot Guys

Thanks fett, for your time and your kindness. I think i see how it's working, so i a lot more in position to solve my problem. ANd the influence of the 2 thermistor.

It's working better with the thermistor of the 3L9. It's not perfect, but after 20/30 km, it's still starting reasonably well, and even if it's not working perfectly, i did'nt remove my spark to clean thme, so i guess they are less dirty than with the other thermistor.

Before i start to nose more, i will wait to have the new coolant thermistor in the range of value and the gas thermistor in place. If i understand well what you say Fett, I think that the engine is running "like with the choke", what could explain for the rich mixture (added with the 25% over value on my bad coolant thermistor). The gas thermistor "in the air" should be around 40°C of value while plugged on the gas, it should be around 80°.

I will check also the tuning of the throttle pot.

I keep you informed guy. Thanks again for you expertise.

Sunny greatings.

Greenislander

Fett, if you pass by West indies (and Dominica), i will be very happy to offer you a drink (or two, or more) in my restaurant.
 
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