Bad Performance of V6

Submitted: Monday, Jan 02, 2006 at 13:02
ThreadID: 29305 Views:1907 Replies:7 FollowUps:3
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This is my first posting, I was wondering if anyone can shed light on my problem.
I have a 1991 V6 4 Runner dual/ fuel which does not go to well when running on petrol, its OK on LPG, but has a real problem exceeding 3000rpm when on petrol, is sluggish, missfires a lot and has no power, if you persevere and keep the gas pedal down to the floor, it leaps into life again around 4500rpm (not that there is much left that high up the rev range ) Low end grunt is also lacking on petrol. I have played around with the air flow sensor & it has improved somewhat, but compared to running on LPG its gutless. The motor has just been rebuilt to gas specs.( valve seat recession caused by the previous owner not ever running it on petrol I recon ) & I have had the LPG system altered to start on petrol, & fitted a Moreys flash lube system to it, but still the problem exists. Any ideas....
Also reading threads on fuel economy of the V6 I have to agree these are thisty suckers, On petrol I get 14.5L/100k country & 17.7 city & on LPG only 19/21 respectively. (on one rather hairy 4 wheel track I managed 32 l/100km, impressed ? )
Still I reckon its a great off roader, haven't found much that will stop it yet.
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Reply By: gramps - Monday, Jan 02, 2006 at 13:53

Monday, Jan 02, 2006 at 13:53

Not being that mechanically minded I can't help you with the first part of your post, however, the second part may give some clues.

I run a 3.4l V6 and can push it up to nearly 14l/100ks fully loaded around 100-110kmh on country runs. In normal suburban/semi-rural (definitely not stop-start city runs) I average around 11.5l/100k. That's at a normal total weight of just over 2,000kgs.
AnswerID: 146310

Reply By: Boo - Monday, Jan 02, 2006 at 13:55

Monday, Jan 02, 2006 at 13:55

We had a very similar problem with our Patrol dual fuel, no problems on gas but sounding almost identical to your problem on petrol.

It ended up being the fuel pump in the tank, it was still working partly, so we were able to drive at low revs, however as soon as it was under load there wasn't enough fuel getting through to power it. We had the pump replaced and now works fine again.

May or may not be your problem, however might be worth a look.


AnswerID: 146311

Reply By: GaryInOz (Vic) - Monday, Jan 02, 2006 at 17:42

Monday, Jan 02, 2006 at 17:42
Any ECU codes??

My theory (FWIW):

Try resetting the ECU, may have gotten itself in a knot. Disconnect the battery overnight and reconnect in the morning. For the next 3-4 tankfulls run on PETROL ONLY. This will allow the ECU to "learn" the operating characteristics of the fuel system/ignition system/emission system on petrol. Most gas fitments to EFI motors have a box (*) that modifies the "point" signal used to fire the ignition coils when using gas.

I'm guessing that what has happened is the battery has been disconnected when they did the installation, and you have used it on gas for most/all of the time the ECU is in it's "learn" mode. LPG burns differently to petrol, and your ECU was inadvertantly callibrated during the learn mode to run on gas instead of petrol (affects ignition timing). Switching to gas switches in the abovementioned box (*) and modifies the timing even further, explaining your poor gas and petrol consumption and poor performance.

Not absolutely sure, but plausible.......
AnswerID: 146324

Follow Up By: Sir_Gibbo - Monday, Jan 02, 2006 at 22:22

Monday, Jan 02, 2006 at 22:22
Good idea except that after the rebuild ( when the battery was disconnected for over a week ) I ran for 1000km on petrol only, so I rekon it should have learnt by then. After that I took the vehicle up to 95 kph then switched over to LPG without moving the throttle, lo & behold it accelerated to nearly 110, so it was definitely running better on LPG.
Before the rebuild & before I started playing around with the LPG system it really was thirsty 25 l/100k so I have improved it a bit.
Found out just before Xmas that the mechanic that rebuit my motor did not clean & recon the injectors, so I think I will do that & see if it all improves petrol performance ( a pig of a job especially the drivers side injectors, but if it has to be done... )
FollowupID: 399890

Reply By: J.T. - Monday, Jan 02, 2006 at 20:53

Monday, Jan 02, 2006 at 20:53
Perhaps the injectors are blocked due to carbon from gas use.If petrol is not used regularly the injectors nozzles block.I agree also the fuel pump or filter(cheapest fix first)could be buggered.
AnswerID: 146343

Reply By: 4runner - Wednesday, Jan 04, 2006 at 17:11

Wednesday, Jan 04, 2006 at 17:11
Hi. I have a 1991 V6 4Runner and find that it is a thirsty little sucker. The best I have had out of it was 12 litres per 100 Km , but on an overall average 13 -14 litres per 100Km is about as good as it gets. Yes, you do have to row the gearbox at low speeds, and on climbing long hills one has to drop from 5th down to 4th. From others I have spoken to about the lack of performance at low speeds , this is normal for a vehicle that weighs in at approximately 2 1/2 tons. Thats nearly 4000 Lbs in Imperial measurement. One thing you should do as others in this forum have suggested, is to replace the fuel pump and also the fuel filter. Failing that then get the fuel injectors checked and cleaned. But for HEAVENS sake if you have to replace them , whatever you do dont buy the injectors from Toyota, as they quoted me $260 per injector. Multiply by 6 and you will see what I mean. Repco or other good quality aftermarket accesory shops can do better than that in terms of price.
Oh and before I forget, have you checked your air filter element????? Its placed in the most ridiculous position, right behind the left hand head lamp and therfore sucks in every conceivable bit of dust and dirt. A snorker would be a better proposition. Hope that has been of some help.
AnswerID: 146409

Follow Up By: Sir_Gibbo - Wednesday, Jan 04, 2006 at 20:41

Wednesday, Jan 04, 2006 at 20:41
You think 13-14 lt/100k is thirsty, I'd love to get that much.....
I'm lucky to get that in MPG.....
I must add that mine is a auto so it will be thirstier
FollowupID: 399953

Reply By: awill4x4 - Wednesday, Jan 04, 2006 at 17:41

Wednesday, Jan 04, 2006 at 17:41
If ever a motor is crying out for an exhaust change to free flowing headers/extractors then the 3.4 litre V6 Toyota motor is it. (don't know about the 3.0 one though)
For a modern twin cam V6 the exhaust on these is an absolute shocker. (Just poke your head under the bonnet of one and look at the exhaust manifold and you'll see what I mean) I would expect considerable gains in both power and economy by a properly designed header/extractor system.
Regards Andrew.
AnswerID: 146413

Follow Up By: Sir_Gibbo - Wednesday, Jan 04, 2006 at 20:30

Wednesday, Jan 04, 2006 at 20:30
I agree a nice set of headers would be nice , any clues on where to get them at a resonable price. I have been looking but unless I want to get them from USA, I have not found an Aussie supplier yet.....
FollowupID: 399948

Reply By: 4runner - Wednesday, Jan 04, 2006 at 22:51

Wednesday, Jan 04, 2006 at 22:51
Hello Sir_Gibbo. What I forgot to mention is that mine has got a 2 3/4 inch mandrel bent stainless steel exhaust sytem from the manifolds all the way to the tail pipe. Thats helped get rid of the exhaust gases a lot quicker - oh and does it sound good !!!! The larger exhaust system has helped improve economy since it was fitted several years ago. A snorkel is the next thing that is going to be fitted in order that the motor can breathe cleaner and cooler air.
AnswerID: 146475

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