manifold pressure at idle

[richardix]

Hello everybody,
Still having issues with rough idling and rough running below 2000 rpm, I measured vacuum and the response. I measured 13 psi vacuum at idle and I was wondering if that is a common number. It's a standard 4 litre hc engine. If I open for a second trottle, vacuum touches 0, goes to 18, settles back to 13 psi. Pen trottle fully vacuum goes above 25 psi and is steady.
So first question is what common vacuum I should expect (I could not find it on the www)


To be sure no headgasket problems I did compression test. All cilinders around 150 psi. Little low so I suspect rings are getting worn

Till 2000 rpm car feels like ignition is too much advanced (hesitating and kangarooing). Allthough ignition is 28 in advance above 3500 rpm without vacuum. According to flywheel. (I know it cannot always trusted, so have not checked the top of piston vs flywheel timing markers. Could also be vacuum leak or maybe worn camshaft. No water consumption at all, no bubbles.

Thanks for your help

[Boosted LS1]

To me that seem's low. It's a long time since I used my vacuum guage but I recall a healthy target being something like 19. Have you got a racey camshaft in there or any air leaks?

[davep]

richardix said:

Hello everybody,
Still having issues with rough idling and rough running below 2000 rpm, I measured vacuum and the response. I measured 13 psi vacuum at idle and I was wondering if that is a common number. It's a standard 4 litre hc engine. If I open for a second trottle, vacuum touches 0, goes to 18, settles back to 13 psi. Pen trottle fully vacuum goes above 25 psi and is steady.
So first question is what common vacuum I should expect (I could not find it on the www)

This thread may help when determining 'correct' vacuum values assuming you're using Lucas distributor, etc: https://www.pistonheads.com/gassing/topic.asp?h=0&...

Interesting that at open throttle you are getting vacuum values of 50+ " Hg, that's a fair bit of advance.

[richardix]

Maybe I was not completely clear. I measured the pressure in the plenum which is 13 hg , not the vacuum of the ignition vacuum advance unit. That should be around 0 during idling. Advance of ignition is working as I see it

[richardix]

It is a normal hc 4.0 car. What I have found the camshaft is probably replaced by a Kent 214

[davep]

Here you go:



So your reading of 13 psi gauge at idle equates to 1.7 psi absolute, which in turn equates to 26 "Hg(v) gauge. This seems a bit high, typical values should be 17-22 "Hg(v). Usually indicates engine trying to suck harder due to restriction in the air intake, blocked air filter etc.

Here's a 123Tune curve diagram showing mechanical and vacuum advance for a standard LR Disco Lucas 35DLM dizzy with ported (I guess) vacuum setup:

Edited by davep on Friday 17th August 17:31

[richardix]

Thanks Dave for your helpfull information. But maybe I have to rephrase my question, because it is regarding the vacuum in the plenum, not the vacuum in the port to feed the timing advance. Timing is ok and also if I measure it over the complete range (with a digital stroboscope) all is working fine. This afternoon measured the physical position of the piston and it is in line with the timing marks, so timing is adjusted ok. Dizzy works fine.

Problem is, Rover Gauge indicates the engine is running rich, the engine is running rough at idle, till approx 2000 rpm. Fuel consumption is bad. Compression is okish, but not great. All temperature gauges are ok (and renewed). complete ignition system is renewed (so new cables, new amp, new plugs, new rotor, new dizzy pickup element, etc). New lambda's. New fuel filter and fuel pump. Checked the AFM (exchanged it with an AFM from a friend) and is good. So I am convinced it is not an "electric" problem.

So did this afternoon a new vacuum test of the plenum (manifold) and found it at idle swinging between 13 and 14 PSI (mercury), above 2000 rpm max 20 Mercury. Stepper motor was disconnected to prevent 14CUX from controlling idle speed. Still idle rpm was swinging between 850 and 930 rpm.

So I am afraid of worn piston rings and/ or worn camshaft. Headgasket is ok in my opinion (no bubbles at high rpm)

[Chimp871]

Extenders? Plug HT leads direct to spark plug. Have you checked for strong spark?

[Boosted LS1]

Fuel pressure regulator vacuum pipe? Doubt it's your rings. 150 psi isn't a bad reading for an RV8. Doubt it's a camshaft issue either. I'd seek a simpler fault that effects 8 cylinders.

[richardix]

Did try without extenders, but did not improve anything. Did also a ht test of the spark. No misfires, although the maximum gap is 2cm. But I replaced coil by bosch type and also replaced the amp. Tried with earthing the engine directly to ground on battery by a temporary cable. Also no improvement.

Last year I did a compression test and it was 160-165 psi at max. So in 5000 km compression decreased.

[richardix]

Fuel pressure was always good. Around 2.7 bar. But I must admit I checked it a few months ago. Will check it tomorrow, good tip.

[SuperApeInGoodShape]

Bit of mix up with the units, InHg & PSI are not interchangable, just like an inch is not a mile, but they are both measuring the same thing.

IF your numbers in the original post are in InHG (inches of mercury) then 13 at idle is in the right-ish ballpark for a 4 litre with a performance cam with plenty of overlap. If your numbers are PSI, then something is very very wrong.

If you don't have a big cam fitted, or the car used to run well, but now doesn't then it's obviously a problem instead of being par for the course.

150psi on a compression test is nothing to worry about in itself, knackered rv8's usually show 80-90 psi on a couple of cylinders. Your 10psi decrease from last time could be a difference in cranking speed, ambient temp, ambient pressure, engine temp, gauge used, etc etc.

Edited by SuperApeInGoodShape on Saturday 18th August 06:38

[ChimpOnGas]

13hg is 45 kPa which is spot on for a 4.0HC at idle.

[Chimp871]

Worth checking your fuel temp sensor, and water temp sensor.

Also worth checking the Afm voltage which is worth a search, blitz racing has covered many times and if you can be bothered voltage on the lambdas too.

150psi seems fine, should always be done on a warm engine, all plugs out and throttle butterfly on max open.

[N7GTX]

Have you checked for vacuum leaks? A poorly running engine below 2,000 rpm could indicate a vacuum leak. Use a spray can of brake cleaner/carb cleaner with a straw fitted to the nozzle. Then with the engine running nice and warm, start with the spray can at one side of the engine and methodically work your way around the plenum. Just small squirts as the spray is likely to be flammable so nowhere near the exhaust. Then around the manifold gasket and listen out for a rise in the revs for a moment or two. Check also all pipes to and from the plenum in the same way including canister (if still fitted).
A leaking brake servo can be eliminated by clamping off the pipe with grips.

[SuperApeInGoodShape]

ChimpOnGas said:

13hg is 45 kPa which is spot on for a 4.0HC at idle.

Or 56kPa I presume you're going off a reading from an aftermarket ECU, does it display WOT/Atmospheric as 0 or 100 (or 101.3)?

[richardix]

You're totally right. I was mixing up units. It is 13 inch mercury .

Just measured fuel pressure. That's 2.6 bar with just fuel pump running. 2.1 at idle. So think that's ok.

Also did trial by connecting coil directly to the battery. No improvement. So no cable problems to ignition. (Earth also fine )

Car ran very well and last year became rougher and rougher during idle and til 2000 rpm.

Fuel temp is functioning as I see the temperature in rover gauge. Between outside temperature and 44 degrees celcius

I checked for vacuum leaks at plenum with spray. Also clamped all hoses for leaks. Could not find anything. Can it be the gaskets from the inlet manifold below the plenum? Or the connections to the cilinder inlet ? Or other gaskets I should try?

Your advice gave me some confidence that the risk on something major is low and the figures till now are nothing to worry about. Thanks for that.

Any other suggestions?

[Boosted LS1]

What do your spark plugs look like, do they all look the same? Are your injectors in good shape?

[SuperApeInGoodShape]

richardix said:

Any other suggestions?

Always difficult to know where to begin & if it's an Engine management issue or a mechanical issue. As a starter for ten, i'd go take a look at a vacuum gauge diagnostic guide, plenty of tests that might help you narrow it down. Then;

Check ignition advance at 500rpm increments with the vac canister disconnected and plugged, might be a case of too much too soon (Advance springs sometimes do come off)

Leakdown test

Whip The valve covers off & check for broken valve springs, hammered collets & cracked caps (the sintered metal retainers are st)

Take off the rockers and pushrods, and use a long probe DTI to measure cam lifts to see if you have lobes with major deviation from the mean.

As for the Engine management, i dunno my way around the 14cux so hopefully someone else can chime in.

[davep]

richardix said:

You're totally right. I was mixing up units. It is 13 inch mercury .

Yep, there's big difference between expressing a value in psiv and "Hg(v)!

richardix said:

Any other suggestions?

Read this: https://www.pistonheads.com/gassing/topic.asp?h=0&...