vacuum leak?

727 Views 34 Replies 8 Participants Last post by rlynch356, 2 mo ago

[email protected]

Discussion Starter · 2 mo ago (Edited)

So I have 3 on going issues on my 94 D90 3.9.

Hard start. Sometimes it can take 10-15 secs to start and sometimes it starts within 3 secs but never right away.
Slight misfire at idle. When idling it will blip a little misfire. Nothing serious and you dont notice it at speed or over 1k RPMs. Additionally, I still get good mpgs. Like 12-13 around city.
Occasionally I throw a code 45. Usually when I am sitting at a light long after it has warmed up.

I have replaced the fuel injectors, fuel pump, fuel regulator and IACV. All of which have helped the improved running of the truck but I still have these 3 nuisance issues. I am guessing a vacuum leak but I am not sure where. All the air hoses including plenum main hose have been replaced with good new hoses and I resealed the plenum split with RTV after pulling it for the injector replacement. Only thing I can think of now is the throttle body cleaning. Anything else anyone can think of?

Edit: I have sprayed all around with starter fluid trying to get an increase in revs but havent found any places that occurs.

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LRNAD90

· #2 · 2 mo ago (Edited)

Smoke machine is probably your best bet for tracking down a vacuum leak. Have you checked the Valve Cover gaskets? My Valve cover bolts seem to always work themselves loose, leading to a whistling sound on acceleration..

I am also assuming you have gone through the electrical side? Quality (as quality as you can get anyway) cap/rotor/wires/ plugs maybe even an ignition amplifier?

[email protected]

Discussion Starter · #3 · 2 mo ago

I think I am good electrical side but never thought of the valve covers. Thanks.

carlosz

· #4 · 2 mo ago

Bill, a little known fact is that silicone rtv and or fumes will often damage and kill an oxygen sensor. you mentioned using such and this could be one your culprits. another could be weak spark due to coil, high resistance on ignition wires and spark plugs including gap and high resistance on distributor rotor.
the ECT acts as a choke and if not reading properly could flood the engine. seems you have covered all else.

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Discussion Starter · #5 · 2 mo ago

Ah interesting. Ill take a look. What should I seal the plenum with if not RTV?

Wires are fat like 9mm I believe and brand new, along with new coil, dizzy and plugs that I gap checked before going in. Also new relocated ignition amp.

I also heard that it could be the valley gasket or at least the intake manifold loosening. But spray all around that area has not caused any revs

WeBeCinYa

· #6 · 2 mo ago

Funny I have same issues on a 3.9 in my RRC, with exception of hard start. I’ve thrown every new part at it, including injectors, 02 sensors, new exhaust, wires plugs, smoke machine, …you name it. It’s now at Jimmy/Drew’s in VA to sort out.

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carlosz

· #7 · 2 mo ago

Hylomar is a sealant recommended by lr, one last point to ponder, ethanol in today's fuel blend and formulation.

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Discussion Starter · #8 · 2 mo ago

Ill order some Hylomar and strip the old stuff. I am going to have to pull the plenum to check the intake bolts anyways.

What to do about ethanol in the gas? It tends to gum up stuff right?

Thanks again Carlos.

[email protected]

Discussion Starter · #9 · 2 mo ago

I’d be interested to see what they find. Mine sat for about 5 years. I am sure there is crud anywhere there could be from sitting there. That’s why I replaced all the fuel line components.

WeBeCinYa said:
Funny I have same issues on a 3.9 in my RRC, with exception of hard start. I’ve thrown every new part at it, including injectors, 02 sensors, new exhaust, wires plugs, smoke machine, …you name it. It’s now at Jimmy/Drew’s in VA to sort out.

Sent from my iPhone using Tapatalk

carlosz

· #10 · 2 mo ago

I have never let ethanol fuel sit for long term including lawn mower fuel. in the vehicle, because it contains alcohol it tends to burn faster thus emulating too advanced timing. Yes it does reduce emissions but creates higher Temps specially in lower compression or worn out engines, the gum can be engine oil fumes from blow by thru piston rings, the more wear the worst blow by.
If pcv fails, then you have excessive pressure in crank case and leaking gaskets and or seals.

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Abrooks

· #11 · 2 mo ago

Do you have the updated ignition module?

dickyboy

· #12 · 2 mo ago

As everyone here knows I'm NO mechanic but when I first got my Defender my "Potentiometer Throttle Control" came loose. I found a guy with a Discovery who had this issue and it fixed mine. Mine ran horribly and did lots of the weird things yours is doing. Worth it to check out and I'm sure you did. I took mine off ( two screws ) and cleaned it and tightened it back on and not an issue since ! I recently had to change my ignition module and that made a world of difference. If you have a friend with one try his before forking out 300 and see if it solves the issue. I'm in Md if I can be of help.

o2batsea

· #13 · 2 mo ago (Edited)

Will echo Tony's suggestion that you replace the ignition module. These are classic symptoms of a bad module. Do not attempt to clean the mass air sensor.
You can detect a vacuum leak by inspecting the spark plugs. They will be very light greyish instead of healthy brown, and the insulator may be clean white. This will indicate a lean mixture condition. You would also get bucking, dying on deceleration or coming to a stoplight, and reduced gas mileage.
I will also add that the engine ground strap should be checked to be sure it has clean tight connection. Low voltage plays tricks on EFI systems.

[email protected]

Discussion Starter · #14 · 2 mo ago

Ignition Module is all good. When I put the new dizzy in I bought a relocation kit and a new module. MPG is good/great and anything above 1k rpms feels like the truck is driving perfect. The misfires only come on idle or low rpms.

Ill give the Potentiometer Throttle Control a look after I figure out what it is. Thanks for all your help.

dickyboy

· #15 · 2 mo ago

[email protected] said:
Ignition Module is all good. When I put the new dizzy in I bought a relocation kit and a new module. MPG is good/great and anything above 1k rpms feels like the truck is driving perfect. The misfires only come on idle or low rpms.

Ill give the Potentiometer Throttle Control a look after I figure out what it is. Thanks for all your help.

It seems as though we should always keep a spare on board. I bought the expensive Module . Do you have a lot of miles on your truck ? Also I purchased a Red Roter! They are 33 dollars but are apparently reliable as hell. Rovers North has them. Throw one of those in. May take away the back firing ? Glad you're making progress!

o2batsea

· #16 · 2 mo ago

It’s running poorly at all rpm you just can’t tell as much

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Discussion Starter · #17 · 2 mo ago (Edited)

No power loss on acceleration. No dead spots. No stuttering. Gas mileage is 12.5 or better. I don't know that it is running poorly at all rpms.

I mean kind of depends on your definition of "poorly," but I would think some of the above would be included.

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Discussion Starter · #18 · 2 mo ago

dickyboy said:
It seems as though we should always keep a spare on board. I bought the expensive Module . Do you have a lot of miles on your truck ? Also I purchased a Red Roter! They are 33 dollars but are apparently reliable as hell. Rovers North has them. Throw one of those in. May take away the back firing ? Glad you're making progress!

I do have a ton of miles but I am not sure how many. The odometer died at 100k and I drove it like 4 more years after that. I have 7k on it since I brought it back to life. I would guess the engine is near 200k. I do have a Red Rotor on mine, but I have never had any backfiring.

rlynch356

· #19 · 2 mo ago (Edited)

So my thoughts...
Code 45 (right side facing it - the drivers side) and the mis-fire are related... and probably the hard start..
The 45 is most likely from the bank not firing on all cylinders some times and thus leaning out the mix on that side Or vice versa with the other bank having the issue and the right side over compensating by adding fuel.
This can be that the O2 is faulty and not allowing it to go into Closed loop when it hits temp or a couple of other things, normally misfires,

Are you sure it is only a Code 45 and not a 44 AND 45 and maybe some others like a fuel temp sender?

The difference is you may have a bank issue or a whole motor issue....

The code reader in the truck only displays the last code... To get the others to come up you have to cycle it, and not disconnect the battery...
Process..
Key on position (not running thought)
Unplug White plug in the ECU compartment - this goes to the Code display unit and have 3 Triangular arranged Pins, This is the series connection plug.
Wait 5 seconds (about how long it takes to get to the key for the next step
Turn Key Off... Wait for ECU relay to drop out
Reconnect Plug
Turn Key on Read next code.

If you get an 02 code it is just the power Interruption code - normal you will get this every time the ECU is disconnected from power for 1 cycle.

If you see a code 59 - that is your culprit... Fuel temp sender - they fail and the engine thinks the fuel is frozen...

But I normally start with replacing the Cap and rotor..if your red rotor is reasonably new its probably fine.. If you have a blue Cap, I'd chuck it for a Lucas one.
I have had Coils, caps, and numerous rotors die.

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WeBeCinYa

· #20 · 2 mo ago

[email protected] said:
I’d be interested to see what they find. Mine sat for about 5 years. I am sure there is crud anywhere there could be from sitting there. That’s why I replaced all the fuel line components.

Mine ended up being broken ground wire on O2 sensor connector that comes from behind the top of the engine on passenger side.

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