Engine missing under load NP 3.8 issue

[Emeraude]

Hi all,

My NP 3.8 is missing and lacks power under load (while driving up a slight hill or on a flat) and am unsure of what is causing this issue.

I've replaced some vacuum lines which were cracked, cleaned the MAF and the throttle body. Drained the fuel as it had been sitting for a while. Put in new fuel and a bottle of injector cleaner into the tank. Changed all 3 coils and installed new Irridium spark plugs. Put in a new fuel pump and filter.

Next, to change the leads on the weekend. I can smell fuel when it's idling but not too bad and no sign of leaks. No significant smoke from the exhaust. Idle is very smooth and when in park, there are no issues when revving it up.

I'm thinking of replacing the injectors but they are over $300 for a set of 6. I'm out of ideas and have been searching online but couldn't find anything.

Any help is greatly appreciated!
Cheers!

[erad]

I had the injectors on my wife's Magna professionally cleaned by a local mechanic. The difference was staggering. Totally different car after the cleaning.

Also, when you remove the spark plugs, by default you disturb the HT leads. Hopefully when you replace them, all will be good.

[Emeraude]

Hi
Possibly spark plug or leads. Could be water in around plugs - happened to me after a pressure wash.
Dan

Hi Dan,

New plugs were installed, leads are fairly new but will replace them anyway. The motor hasn't been washed and is dry.

Cheers!

[Emeraude]

I had the injectors on my wife's Magna professionally cleaned by a local mechanic. The difference was staggering. Totally different car after the cleaning.

Also, when you remove the spark plugs, by default you disturb the HT leads. Hopefully when you replace them, all will be good.

Erad,
I decided to buy new injectors rather than have them serviced as the cost is nearly the same here. Will put new leads in as well since the manifold is off.
Cheers!

[MACXR8]

Most places should charge between 25-35 per injector to clean once removed from car

[erad]

Emeraude:
If you still have problems after replacing the injectors, look at the Oxygen sensor in the exhaust. If that is screwy, you may be getting mixtures which are too lean and that can cause misfiring.

[Emeraude]

Most places should charge between 25-35 per injector to clean once removed from car

The places in SE Qld are asking $45-$50 each.

[Emeraude]

Emeraude:
If you still have problems after replacing the injectors, look at the Oxygen sensor in the exhaust. If that is screwy, you may be getting mixtures which are too lean and that can cause misfiring.

Well, I replaced the spark plug leads and injectors but it's still missing and backfiring in drive. It will run smoothly in Neutral. So, it looks like I will replace the Oxygen Sensor next...I don't know what else could be causing this.

[erad]

Have you had the intake manifold off lately? If so the engine may be running too lean if there is an air leak somewhere. A lean mixture leads to backfiring.

Also, silly question, but did you put the new leads onto the correct plugs? I replaced the leads on my NL 3.5 L engine and the thing would run but no power, and it backfired badly on LPG. I tried everything I could think of and traced the HT leads. About 65 mm back from the coils was a number cast into the intake manifold. My wife said "Does that have anything to do with this problem?". I said 'No - that would be a part number for the manifold. Eventually, I looked at it and it was something like 521463, and it dawned on me that maybe that was the HT lead numbers, 52 coil being for Nos 2 and 5 cylinders, 14 for cylinders 1 and 4 etc. Carefully removed the manifold again and rerouted the HT wires to this 'new' configuration, and Voila! - the engine ran perfectly. It took a long time for me to live that down... My logic was totally different to the designer's as to where to locate the coils. Each coil on my NL (I imagine the same for yours) fed 2 cylinders and the firing order is 1,2,3,4,5 and 6, so 1 and 4 share the same coil, 2 and 5, and 3 and 6 are the same. Check the HT lead location.

Also, make sure that the leads do not touch either the engine or themselves. The insulation is very thin on these leads and it is easy for them to short to earth or cross fire and thus cause a backfire. Be meticulous as to how you run the leads. NGK leads were the best to use because they were numbered, and you got the exact length for each lead. I tried different brands and all were too long, resulting in the leads sagging either onto the rocker cover or onto another lead, but NGK were exact. They were the original fitment as well

[Emeraude]

Have you had the intake manifold off lately? If so the engine may be running too lean if there is an air leak somewhere. A lean mixture leads to backfiring.

Also, silly question, but did you put the new leads onto the correct plugs? I replaced the leads on my NL 3.5 L engine and the thing would run but no power, and it backfired badly on LPG. I tried everything I could think of and traced the HT leads. About 65 mm back from the coils was a number cast into the intake manifold. My wife said "Does that have anything to do with this problem?". I said 'No - that would be a part number for the manifold. Eventually, I looked at it and it was something like 521463, and it dawned on me that maybe that was the HT lead numbers, 52 coil being for Nos 2 and 5 cylinders, 14 for cylinders 1 and 4 etc. Carefully removed the manifold again and rerouted the HT wires to this 'new' configuration, and Voila! - the engine ran perfectly. It took a long time for me to live that down... My logic was totally different to the designer's as to where to locate the coils. Each coil on my NL (I imagine the same for yours) fed 2 cylinders and the firing order is 1,2,3,4,5 and 6, so 1 and 4 share the same coil, 2 and 5, and 3 and 6 are the same. Check the HT lead location.

Also, make sure that the leads do not touch either the engine or themselves. The insulation is very thin on these leads and it is easy for them to short to earth or cross fire and thus cause a backfire. Be meticulous as to how you run the leads. NGK leads were the best to use because they were numbered, and you got the exact length for each lead. I tried different brands and all were too long, resulting in the leads sagging either onto the rocker cover or onto another lead, but NGK were exact. They were the original fitment as well

Yes, intake manifold came off when I replaced the injectors and leads.
Yes, I followed the same way they came off when I replaced them, one at a time.
Will check the leads again tomorrow and also the firing order. Pretty sure they are the same as the came off. I bought a set of EAGLE IGNITION LEADS.
Thanks again for all the help!

[Emeraude]

Yes, intake manifold came off when I replaced the injectors and leads.
Yes, I followed the same way they came off when I replaced them, one at a time.
Will check the leads again tomorrow and also the firing order. Pretty sure they are the same as the came off. I bought a set of EAGLE IGNITION LEADS.
Thanks again for all the help!

Update:

After replacing the Oxygen sensor and also finding a tiny crack on the intake plastic tube thing that fits onto the intake manifold from where the air filter is located, the combination of both a vacuum leak and a faulty O2 sensor seems to have caused the missing and lack of power.
The problem has now been fixed! The engine is running smoothly and drives well.

Special thanks again erad for all the help!