Rough idle - how smooth should it be?

[Brianmoooore]

Fuel pressure on a 2.5 should be 3.0 bar (written on the FPR), BUT that's 3 bar across the injectors, and one end of the injectors is in the partial vacuum of the inlet manifold, which is at its strongest at idle. The small hose from the inlet plenum to the FPR transmits this partial vacuum to the FPR. You, however, are measuring fuel pressure relative to the atmosphere, which is where the 0.5 bar is disappearing. The temporary increase in pressure when you blip the throttle is caused by the air you let in increasing the pressure inside the manifold, before the increased revs reduces it again.
Pull the small hose off of the FPR, and you should see a constant 3.0 bar at all throttle openings.

[BristolE30]

Fuel pressure on a 2.5 should be 3.0 bar (written on the FPR), BUT that's 3 bar across the injectors, and one end of the injectors is in the partial vacuum of the inlet manifold, which is at its strongest at idle. The small hose from the inlet plenum to the FPR transmits this partial vacuum to the FPR. You, however, are measuring fuel pressure relative to the atmosphere, which is where the 0.5 bar is disappearing. The temporary increase in pressure when you blip the throttle is caused by the air you let in increasing the pressure inside the manifold, before the increased revs reduces it again.
Pull the small hose off of the FPR, and you should see a constant 3.0 bar at all throttle openings.

Here’s a video of it underload in neutral. You can see it goes up to about 2.7bar when I blip, then drops to almost under 2.5 bar. Sorry about the awful video you can just about make it out. So really when I rev it, it should be 3 bar you saying?



Also a photo of my spark plugs confirms lean symptoms

.

[Brianmoooore]

So really when I rev it, it should be 3 bar you saying?

It would be 3 bar IF you could rev the engine while it was under load, where you could keep the throttle wide open without the revs going through the roof. The way to simulate the engine being under load with the throttle wide open, is to pull off the vacuum reference pipe, as I said in my last post.
The plugs indicate a weak mixture, but we already knew that. What we don't know is why, but it looks like fuel pressure problems can be eliminated from the hunt.

[BristolE30]

Pull the small hose off of the FPR, and you should see a constant 3.0 bar at all throttle openings.

I will try this tomorrow to confirm it reaches 3 bar

[BristolE30]

So really when I rev it, it should be 3 bar you saying?

It would be 3 bar IF you could rev the engine while it was under load, where you could keep the throttle wide open without the revs going through the roof. The way to simulate the engine being under load with the throttle wide open, is to pull off the vacuum reference pipe, as I said in my last post.
The plugs indicate a weak mixture, but we already knew that. What we don't know is why, but it looks like fuel pressure problems can be eliminated from the hunt.

Also even if fuel pressure does go up to 3bar with the vac pipe disconnected, surely that just proves the FPR is working? The fuel pump could still be weak and proving not enough pressure under normal operation?

Let me know thoughts, I may try connecting the gauge to the pump from the rear seat end and driving around to see how it looks under load

[BristolE30]

Vac line off fuel pressure rose to 3 bar.

No idea where to go from here

[Brianmoooore]

Been to the garage to check out AFM numbers. Looked at the AFMs in three 325is, and each had identical numbers on the lid of the pot. box - 0 280 202 082 - 1 286 615 9.
Looked at one 320i (bonnet wouldn't release on the other), and the numbers are 0 280 202 083 - 1 710 539 9

[BristolE30]

Been to the garage to check out AFM numbers. Looked at the AFMs in three 325is, and each had identical numbers on the lid of the pot. box - 0 280 202 082 - 1 286 615 9.
Looked at one 320i (bonnet wouldn't release on the other), and the numbers are 0 280 202 083 - 1 710 539 9

Yes mine is ending in 082 also.

I checked the number stamped on it also and it says 17.7. Checked the screw height and it matches this. Currently doing another smoke test to see if I can find anything I missed

[BristolE30]

Okay so second smoke test was a little more successful. I think the first time I didn’t pump it enough or allow enough time.

First major leak was around the breather hose on rocker cover side. I think there probably was always a pin hole leak, but made significantly worse earlier this week when I took throttle body off as it cracked a little more. Probably the reason my idle jumped to 900rpm from 600rpm, and not actually the throttle stop screw I was adjusting, as mine is barley touching the butterfly anymore. Clears that up as that was confusing. Either way I’ve ordered a new hose and clamp.

Second leak was a smaller one on the throttle body, some sort of bearing I’m guessing? Anyone ever repaired this? This probably is the reason my idle is hunting

[Cloggy Saint]

The part circled in red is the elbow that goes from ICV to TB (but you knew that!). It is a splined fit but the splines can wear away very quickly which leads to a dodgy fit & seal. I used a few drops of adhesive on mine and it's held up over the years.

[BristolE30]

The part circled in red is the elbow that goes from ICV to TB (but you knew that!). It is a splined fit but the splines can wear away very quickly which leads to a dodgy fit & seal. I used a few drops of adhesive on mine and it's held up over the years.

Okay yea I was thinking maybe some gasket sealer or something!

I’ve been searching to see if there are any parts for the ICV metal elbow but it doesn’t look like it. I’m guessing BM would just replace the whole throttle body back in day.

I did see one guy tap it out and put a plumbers elbow on it.... worth a thought

[paultv]

I found the best thing is white wood glue - cheap stuff in a squeezy bottle -

Paul

[Brianmoooore]

Your fuel pressure tests have shown you that there's minus 0.5 bar of pressure inside the manifold at idle. (3.0 bar - 2.5 bar).
At that sort of pressure difference, quite a bit of air is going to be sucked in through those leaks.
There's the ICV elbow on one side, and one or two elbows on the other side for the brake servo. These are steel elbows, pressed into the alloy, and if any show any sign of being loose, they need to be removed, cleaned, and glued back in with something like araldite.
There is also a small unused port underneath the throttle body, very close to the TPS, which should be blocked off with a rubber cap. This cap is made from a kind of rubber that deteriorates, and most will have fallen off by now.

[BristolE30]

Your fuel pressure tests have shown you that there's minus 0.5 bar of pressure inside the manifold at idle. (3.0 bar - 2.5 bar).
At that sort of pressure difference, quite a bit of air is going to be sucked in through those leaks.
There's the ICV elbow on one side, and one or two elbows on the other side for the brake servo. These are steel elbows, pressed into the alloy, and if any show any sign of being loose, they need to be removed, cleaned, and glued back in with something like araldite.
There is also a small unused port underneath the throttle body, very close to the TPS, which should be blocked off with a rubber cap. This cap is made from a kind of rubber that deteriorates, and most will have fallen off by now.

This is true. I’ve bought some araldite so will try it. I’m guessing I’ll just have to redo it each time I take the ICV off - it’s supposed to move up and down right, the elbow?

So new breather hose arrived, put it on and seems to have calmed the hunting. Also put some tape on the elbow for temporary measures.



However as can be seen in video it seems my throttle butterfly won’t close enough to achieve a steady idle. It idles at about 1000, unless I push it back and let it go, it drops to around 750 perfectly. But as soon as I hit the accelerator pedal it jumps back up to 1000.

The throttle stop is not touching the butter fly mech as you can see. Do the springs just wear out? What’s the reason it won’t go back further... could it be when I reset the TPS? I didn’t think the TPS had an effect on the position of butterfly. And yes TPS is reading continuity

[boiliebasher]

Throttle cable adjusted too tight?!

[tha881]

Are you sure your throttle cable is not stopping the butterfly returning to the stop bolt?

[boiliebasher]

Are you sure your throttle cable is not stopping the butterfly returning to the stop bolt?

Hahaha! Got in there one minute before your post lol. Great minds think alike and all that

[Brianmoooore]

In the video clip, you can see the mech. pulling on the throttle cable when it is forced against the stop. Check that there's a small amount of slack in the cable by backing off the plastic nut and its locking nut. Also check that at full throttle, the butterfly is fully open, but not being forced. Adjust the stop beneath the throttle pedal to suit.
Connector for the TPS doesn't need to move. Point it the right way, and Araldite it up.

[BristolE30]

I wish! Just went and had a look and there is actually some slack in the throttle cable.

It’s like the spring can’t push it that tiny bit more back, and as Brian mentioned earlier the idle shouldn’t really be reliant on the butterfly, so I don’t think it’s supposed to be opening that last bit!

[BristolE30]

Could it be that the springs are gunked up? They are really dirty looking

[Brianmoooore]

Give the whole lot a good spray of WD40 and 'exercise' it.

[BristolE30]

Give the whole lot a good spray of WD40 and 'exercise' it.

Gave it a good blasting of carb cleaner and then WD40 - seems to have freed it up a little, and after checking inside the butterfly is closed.

I am now wondering however if the high idle is to do with my thermostat. Since I've owned the car it has run cool, and it doesn't often get up to 1/4 mark (325i). I've never been to bothered by it as I'm more scared of overheating. It will however get up to 1/4 if you idle for a little while - and I think I've noticed the idle drops to 750 when it hits the 1/4 mark after idling.

Am I correct in thinking coolant temperature relates to the cold start function? ie if coolant temp is low, injectors/extra injector fires more fuel?

Could be completely wrong but just a thought

[Brianmoooore]

Yes, engines run on gas, not liquids, so petrol is a poor choice of fuel that has to be made to simulate a gas for an engine to work. This is done by spraying the petrol into a fine mist by the injectors, but, f the surrounding surfaces are cold, it quickly starts to condense back into larger droplets that won't fuel an engine. To counteract this, extra fuel is turned into a mist, so that enough survives for the engine to run. As the engine heats up, less petrol condenses out, and the amount of extra fuel injected is reduced/
The excess petrol first of all removes the oil film from the cylinder walls, causing wear, and then ends up in the sump, diluting the oil and reducing its lubricating properties.
From the above , you can see a) why I don't run any of my cars in daily use on petrol, and b) that you should sort out your cold running problem ASAP.
A new thermostat is a good and cheap place to start.

[BristolE30]

Yes, engines run on gas, not liquids, so petrol is a poor choice of fuel that has to be made to simulate a gas for an engine to work. This is done by spraying the petrol into a fine mist by the injectors, but, f the surrounding surfaces are cold, it quickly starts to condense back into larger droplets that won't fuel an engine. To counteract this, extra fuel is turned into a mist, so that enough survives for the engine to run. As the engine heats up, less petrol condenses out, and the amount of extra fuel injected is reduced/
The excess petrol first of all removes the oil film from the cylinder walls, causing wear, and then ends up in the sump, diluting the oil and reducing its lubricating properties.
From the above , you can see a) why I don't run any of my cars in daily use on petrol, and b) that you should sort out your cold running problem ASAP.
A new thermostat is a good and cheap place to start.

Ah yes... that sounds less than helpful for my engine. So do the injectors spray for longer then? How does it actually work? I know some cars have an extra injector for cold start but I don’t think mine does.

The only thing that confuses me about this is wouldn’t this result in a rich mixture? And my spark plugs are white hot?
Thanks for the tips Brian

[Brianmoooore]

The injectors spray for longer, but as I wrote in my last post, the extra fuel never makes it to being ignited by the spark plugs - it just causes bore wear and degrades your oil.
The only time the extended pulse to the injectors will make the plugs go black is when extra fuel is still being injected after the surfaces that the petrol condenses out on are hot.

[BristolE30]

The injectors spray for longer, but as I wrote in my last post, the extra fuel never makes it to being ignited by the spark plugs - it just causes bore wear and degrades your oil.
The only time the extended pulse to the injectors will make the plugs go black is when extra fuel is still being injected after the surfaces that the petrol condenses out on are hot.

So you are saying it’s possible for the engine to be overcooling whilst still running lean from a vac leak or something?

[Brianmoooore]

Entirely possible. It's extremely bad for an engine running on petrol to run cool. If the engine has a lambda sensor, it will sense the excess fuel and lean it off a bit to try to protect the cat., and the ECU will turn on the engine managment light, but no luxuries like that on most E30s.

[BristolE30]

Update and recap: So remember how I was saying I had that strange idle problem a while ago. Really low idle and engine would stall on the downward revs sometimes. Well as said in previous posts one day I took the throttle body slightly off to see if I could reset the TPS and give it slightly more room before the click out of idle circuit. (I was worried maybe it was not fully on idle circuit) tested and the made sure it was correct with meter. So after I did that I reconnected it all and idle instantly shot to 1000rpm. It had always been about 550 since I owned it. So then I smoke tested it and found that breather hose was leaking bigso I replaced that. Idle then dropped to a smooth 750. At least for a month. Then I did a coolant change a couple weeks ago and it started to do the stalling thing and 550 rpm. Did another smoke test and found a tiny tiny crack on the intake elbow so I ordered a new one and replaced it. When I turned on engine it was the same and seemingly nothing had changed. It nearly stalled pulling out of garage. So when it was out of garage I was like whatever I need to replace license plate lights anyway, so I disconnected battery and did that. When I started the engine back on the oil light came on which it wasn’t before. I checked the oil and it was at half, which I thought was weird seeing as it was full a couple months ago and can’t see any leaks on floor. Anyway topped it up, started engine again, oil light went off and idle is now perfect again at 750rpm. I am completely dumbfounded as to why this happened.

Did I reset the ECU by disconnecting battery? Was the ECU messing with the idle and also not showing the oil light?

Anyway for now, the problem seems to be sorted and I have a smooth 750. If something electronic is failing, it may resurface again. Who knows. Thank you for everyone who input’ed so far, it has been so so helpful!

[tha881]

Pleased you've got it sorted. The oil light on mine comes on when the dipstick is showing half way between the lines. Disconnecting the battery does reset the ecu, but I think the idle settings can change over time. It may be the boot was the issue but the electronics were slow to adapt to the fix, hopefully you get my meaning.

[BristolE30]

Pleased you've got it sorted. The oil light on mine comes on when the dipstick is showing half way between the lines. Disconnecting the battery does reset the ecu, but I think the idle settings can change over time. It may be the boot was the issue but the electronics were slow to adapt to the fix, hopefully you get my meaning.

Yea that makes sense, maybe I didn’t give it enough time to adapt. It’s odd that the oil light wasn’t registering until I reset the ECU also

[BristolE30]

Annoyingly after 24 hours of great running the issue has returned. I then reset the ECU again by disconnecting the battery and it ran great for another 24 hours or so before suddenly returning to the low idle again.

Does anyone have an idea what this might be? Or has anyone experienced this before?

Could it be a bad ecu? Resetting ecu it makes it run great for a while, until it just decides to not. I do understand the ECU has a learning period.

Here is another video. Some thing I noticed was that this was on a completely cold start up and the idle was low immediately. Usually when it’s working it hangs higher for a bit due to cold start function. It does not now.

[BristolE30]

Could this be the issue!? Just read in the E30 restoration bible.

I have an early 88 and a 173 ECU.

[tha881]

The ecu is a possibility, you can use a 320i one and swap your 325i chip in it. If you try a different one, do some research and find the latest 320i ecu that was available

[BristolE30]

The ecu is a possibility, you can use a 320i one and swap your 325i chip in it. If you try a different one, do some research and find the latest 320i ecu that was available

Yea I’ve been looking at a few used 380’s as I think they were the latest!

[Banban3]

The ecu is a possibility, you can use a 320i one and swap your 325i chip in it. If you try a different one, do some research and find the latest 320i ecu that was available

Yea I’ve been looking at a few used 380’s as I think they were the latest!

I have both numbers: originally 380 and a spare 173 I have run with for a significant time, and the only reason why my idle would be somehow erratic (not seen on the needle but feeling the car bouncing) was when the lambda was disconnected and the AFM mixture screw not screwed deep enough (hence idle running too lean). Adjusting the screw would instantly make my idle real smooth and stable, still with the 173 ÉCU.

Now I have some other issues so can’t confirm if there is a difference between the 173 and the 380, but I did not feel any so far.

I must admit I did not go through the whole post, but from your latest messages: do you have a lamba sensor? If so, have you connected it? Asking because I wonder if your problem could come from a faulty lambda so the ÉCU would adjust to an erroneous input. And indeed, when you disconnect the battery long enough you clear the lambda adjustments, which could explain why idle works fine for a limited time after reconnecting the battery, but not on a long run.

Other test possible: disconnect the battery to remove all lambda readings, disconnect the lambda sensor, reconnect the battery and see if idle remains stable after a correct adjustment of the mixture screw on the AFM. This adjustment I key to having a stable idle when running without a lambda.