P38A Very high idle

Hi everyone,
I need your help again!

Over the last month or so I noticed a very slight misfire on idle. Came back from camping after easter (400 mile round trip) and didn't miss a beat, until I stopped outside a newsagents while the Mrs popped in. The rangie (running on lpg) cut out, as if out if fuelse but started up straight away. This (lpg being sensitive to ignition problems) and the slight misfire pointed me in the direction of the plugs and leads. Over the course of the week (being on nights) I've changed all the plugs and leads.

The plugs themselves didn't look too bad, all showing the dame wear and tear consistently across all cylinders. The leads looked original, 1 and 8 were completely shot (the bit that goes on to the plug had most of the metal missing) whilst the other end weren't bad.

Started her up and it shot straight upto 2500rpm, so I cleaned out the throttle body and that sensor and same again, high idle. I've double and triple checked everything I touched and everything is connected, having said that I could have disturbed a pipe or something by the ignition pack.

In short, normal idle before plug and lead change, high idle afterwards. Any ideas?

Many thanks

Do you have any diagnostics? I would say you might need it.

Thank you for the reply. I dont, only thing I have is an obd Bluetooth reader which doesn't work on her. I should have said it's a '98 4.6 gems.

OK, so I would be looking at things that would tell the ecu to up the revs.
Prime candidate I would think to be temperature sender. When engine is cold, the idle speed is higher so if the temp sender is open circuit, it'll be telling the ecu that the engine is at "minus many degrees C" so it will up the revs.
Other things would be throttle potentiometer, air leaks (possibly but would think the ecu would cope unless it was really bad) aircon system can also up the revs but wouldn't have thought by that much.
Can't think of anything else off the top of my head.
If this is a daily car, I would seriously think about getting a nanocom, it will pay for itself in saved headaches alone within a year. They ain't cheap but vest bang for your buck possible on a p38!

are the fans coming on for air con? Maybe a coolant sensor?
does it settle down at all?

Wazzajnr said:

OK, so I would be looking at things that would tell the ecu to up the revs.
Prime candidate I would think to be temperature sender. When engine is cold, the idle speed is higher so if the temp sender is open circuit, it'll be telling the ecu that the engine is at "minus many degrees C" so it will up the revs.
Other things would be throttle potentiometer, air leaks (possibly but would think the ecu would cope unless it was really bad) aircon system can also up the revs but wouldn't have thought by that much.
Can't think of anything else off the top of my head.
If this is a daily car, I would seriously think about getting a nanocom, it will pay for itself in saved headaches alone within a year. They ain't cheap but vest bang for your buck possible on a p38!

Click to expand...

Is that the coolant temp sensor at the back of the engine? I find it weird that it was fine before changing the plugs and leads, can only think I disturbed something without realising it. It's not my daily driver thankfully, have looked at nanocam but the price shot up!

Mark Piercy said:

are the fans coming on for air con? Maybe a coolant sensor?
does it settle down at all?

Click to expand...

Never seen of heard them come on, having said that the air con has decided to stop working so will have to look at that (compressor spins freely so isn't seized.)
Ran it for 10mins and revs started around 2200 then gradually increased to around 2800

on some cars they'll come on if a coolant sensor fail to protect the engine from overheating. Some not all. Just to rule stuff out really..
does it run the same on and off the lpg?

2800 is awfully high on a v8.

Remind me, is it a throttle cable on the v8? If so, could it be sticking?

Grrrrrr said:

2800 is awfully high on a v8.

Remind me, is it a throttle cable on the v8? If so, could it be sticking?

Click to expand...

good point grrrrrrrrrrrrrgrrrgrgrggrgrr!

Grrrrrr said:

2800 is awfully high on a v8.

Remind me, is it a throttle cable on the v8? If so, could it be sticking?

Click to expand...

I've tried the cable from both the throttle body end and the pedal end and seems to move freely. I'm ging to take the air pipe off and have a good look at the throttle body (butterfly bit) tomorrow and see if that is closing fully.

Thanks for all the suggestion so far, keep em coming

OK, so, the idle starts at 2200 rpm then slowly rises to 2800 rpm, has to be temperature related then.
Some temp senders exhibit negative coefficient when they fail. Also, a high resistance connection will get worse as it gets hot so that tallies up too.
Really need to get this on diag

Wazzajnr said:

OK, so, the idle starts at 2200 rpm then slowly rises to 2800 rpm, has to be temperature related then.
Some temp senders exhibit negative coefficient when they fail. Also, a high resistance connection will get worse as it gets hot so that tallies up too.
Really need to get this on diag

Click to expand...

Thanks for your continued help! It does seem to be pointing in that direction doesn't it? As you say, diagnoatics is the way to go now.

EDIT: where is the temp sender located? Coolant temp sender? thanks

P38Penguin said:

Thanks for your continued help! It does seem to be pointing in that direction doesn't it? As you say, diagnoatics is the way to go now.

EDIT: where is the temp sender located? Coolant temp sender? thanks

Click to expand...

Have you downloaded RAVE yet? All the details will be in there on location of sensors and wiring diagrams etc.

Hi guys, thought I'd give you a quick update.

Checked and cleaned MAF, throttle sensor and that made ono difference. Had a look on RAVE and got distracted (as you do lol) by the cruise control section (hasn't worked since I've had her. Went out to have to have a poke and prod and noticed the cruise cable was a bit like a banana. Had a poke at the throttle end and noticed there was around1mm of travel from the supposed closed position to the actual closed position. Loosend the cruise cable slightly and hey presto, idle is now back to normal.

The clips on the cruise cable have all snapped off so will have to get a new one ordered but thankfully a nice easy fix!

Wow, as simple as that in the end. Good news and, I guess, a good lesson for us all.
"never overlook the obvious "

If instructions in RAVE are followed reference setting of cruise control cable this can never happen.

Fat fingers on the cables, eh?

GEMS has a vacuum tube for the cruise. Chances are if you replace that with some silicone tube it'll be good to go for years.

wammers said:

If instructions in RAVE are followed reference setting of cruise control cable this can never happen.

Click to expand...

Rest assured I'll set it up according to RAVE

Grrrrrr said:

Fat fingers on the cables, eh?

GEMS has a vacuum tube for the cruise. Chances are if you replace that with some silicone tube it'll be good to go for years.

Click to expand...

Please, I prefer the term 'portly'
I have some red vac pipe specifically for that task! I've traced the vac hose across the bulkhead and, with the help of an endoscope, found where it disappears behind the coolant pipes to the heater matrix, but after that I loose it and cannot find where it goes through the bulkhead, or the second brake light switch for that matter. Looks like a good yoga session in the footwell is order!

@MrGorsky did his recently so can probably advise where it goes. Once through the bulkhead you can probably tape it to the old one and just pull it through?

My new vacuum pipe was bigger than the old pipe so I just "lost" the grommet it went through, and regretted it for the next 800 miles in very cold weather because I had freezing feet.
If you replace the pipe, make sure you seal it well or you'll regret it in winter!
Never knew so much cold air could fit through such a small hole!