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Troubleshooting rough idle
by
Puddleglum
on 19 Aug, 2022 13:22
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Hello all, I’ve been lurking the forum and silently getting answers since January as I’ve been repairing my Rabbit. I have really appreciated all that you guys share with the community. Unfortunately, I am running in to a rough idle issue that I have been unable to solve based on the existing threads I can find.
The car in question is a 1980 Rabbit with 1.5L IDI. After purchase, I drove it ten miles to a place I could store it. The temp gauge did not work at the time and I since have found it had cooling system issues. (It is likely to have overheated) For the last mile, I noticed it would shake violently at idle. Since then, while I have fixed many other issues, the shaky idle remains. Below about 1000rpm, the car violently rattles and the dash is visibly displaced about a quarter inch.
If anyone could help point me in the right direction for what to investigate it would be much appreciated. I am an aircraft mechanic and started this as a personal hobby project to learn the guts of an old diesel. I have no prior diesel experience.
WORK DONE:
Replaced head gasket when it failed recently
Replaced timing belt
Installed new injector nozzles and sent them out for pop-testing and balancing
New rubber fuel lines
New fuel filter
New engine mounts (except mount under radiator which looked fine)
Engine “aligned” per Bentley manual
I don’t have dial indicator adapter so timed by “ear”
OTHER NOTES:
-Before head gasket blew, I was getting compression overall just below spec with two cylinders lower (I forget how much, something like 10-15 lower). I am planning to recheck when I can borrow the tool.
-When I changed the gasket, I found the old was a four notch and I measured for and installed a three notch gasket.
-Cylinder walls and valves all looked visually good to my untrained eye while it was apart. No scoring or burning. I do not have a perfect straight edge, but by comparing various checks, I do think the head is a bit warped. Maybe 4 thousandths? I can’t be sure with the straightedge I have
-Since head gasket, now engine starts more easily. Always caught instantly if warm, but now with glow plugs it starts quickly cold. It hated cold starts before.
-The technician at the diesel shop says to get the pump rebuilt, but that’s $700 to him so he would be biased. I also have heard the pump rarely goes bad.
-When timing, I can adjust the pump through its full range of motion without affecting sound significantly. With less advance, I get white smoke and sluggish response. More advance takes the smoke away and improves the responsiveness. I never hear the diesel sound get much more harsh and the cold start doesn’t seem to change anything.
-when returning to idle, the engine hangs at 1500-2000 rpm for an extra second or two on its way down
-twice on the last test drive, it threw a single puff of black smoke seemingly at random during medium acceleration
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#1
by
ORCoaster
on 19 Aug, 2022 22:33
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Welcome Puddleglum, Even Lurkers finally come out of the dark to get additional help.
So ya gots a 1.5L eh? Not so bad, some say the ideal Classic Rabbit engine. After all, it was #1.
So let's go down the list of work you have completed and see if there is anything additional to do.
WORK DONE:
Replaced head gasket when it failed recently Did you check that head for proper tolerance of like .004 thou of an inch?
Replaced timing belt Always a good thing on an unknown mileage or serviced engine.
Installed new injector nozzles and sent them out for pop-testing and balancing. OK, so system delivery is solid from that point on but IP? I will come back to this if you haven't listed it below.
New rubber fuel lines Good to have
New fuel filter A must to have.
New engine mounts (except mount under radiator which looked fine) Ahh, perhaps it is not as good as it needs to be, or is hard as a rock so it doesn't absorb any vibration?
Engine “aligned” per Bentley manual OK, that is generally a miss for most folks.
I don’t have dial indicator adapter so timed by “ear”. Ahh, this is not going to work. Get a dial gauge so you know your numbers as old engines may have less pressure in their IP pumps and even running the right # of mm of advance they can still be out of time with the performance. Knowing where you are at for a starting point is a gotta have. At least in MHO.
OTHER NOTES:
-Before head gasket blew, I was getting compression overall just below spec with two cylinders lower (I forget how much, something like 10-15 lower). I am planning to recheck when I can borrow the tool. The addition of a new gasket may have helped get you back to spec but maybe not. 1980s motor, may need a new set of rings to be close to spec and if you live in a cold climate you will need that compression to get it to fire up when below 20 F.
-When I changed the gasket, I found the old was a four notch and I measured for and installed a three notch gasket.
If you measured that is kudos to you for doing that. Protrusion is so important with that diesel engine.
-Cylinder walls and valves all looked visually good to my untrained eye while it was apart. No scoring or burning. I do not have a perfect straight edge, but by comparing various checks, I do think the head is a bit warped. Maybe 4 thousandths? I can’t be sure with the straightedge I have. Ahh, so you did check and are not sure about how good it was, that answers my previous comment on that item. A compression check should verify any leaks so get the tools back.
-Since head gasket, now engine starts more easily. Always caught instantly if warm, but now with glow plugs it starts quickly cold. It hated cold starts before. So you gained some compression, but is it balanced or just making starting easier. Tools and check, please.
-The technician at the diesel shop says to get the pump rebuilt, but that’s $700 to him so he would be biased. I also have heard the pump rarely goes bad. Ok here is the deal I know on IP's. Designed to use thicker fuel in the 80's. That is not available anymore so to get it back there are additives you can put in the tank to make it have the lube and the viscosity it should have had in the past.
If you really want to try something quick and have a spare 1-gallon fuel can around try this. Go to the store and find some of that canning wax. Do you know the bricks of paraffin? Put about 3/4 of a gallon of diesel in the can and set it in the sun to warm it up some. While that is happening take half of one of those pieces of wax and shave it into thin slices with a knife. Don't cut yourself. Now put the shavings into the fuel and let them dissolve into it. It takes time but it does happen. You have just created a super additive mix that you will now pour into your tank.
This fuel mix contains lube, a thickener, and what clinches the deal is CETANE! That is what a diesel likes to run on.
Old IPs also wear out, and that affects the timing and the advance. The internal pump pressure may be down too and so the whole system can be out of spec. There are simple tests and devices that you can add to check the health of the IP you have. Rebuilding mostly replaces the gaskets and cleans the internals. Rebuilding pumps to spec is a very complex job and most of the wear can be dealt with by adjusting some parts to compensate for the wear.
-When timing, I can adjust the pump through its full range of motion without affecting sound significantly. With less advance, I get white smoke and sluggish response. More advance takes the smoke away and improves the responsiveness. I never hear the diesel sound get much more harsh and the cold start doesn’t seem to change anything. This all sounds normal to me. But without a dial gauge, you are chasing butterflies, my man. If you could measure the full range you have then you would know how retarded and advanced you are adjusting it. The harsh sound of a diesel may be related to the poor adjustment of valve lash and not necessarily advanced timing.
-when returning to idle, the engine hangs at 1500-2000 rpm for an extra second or two on its way down I think this is a condition of a poorly adjusted or worn throttle bushing. My pea brain is failing me at the moment.
-twice on the last test drive, it threw a single puff of black smoke seemingly at random during medium acceleration
This sounds wrong if you have new injectors in the block. Maybe some diesel injector cleaner in the current tank of fuel. Or that wax mix will do some cleaning of the fuel system too. Delivery valves can stick too, you know the ones at the end of the head of the IP.
This should be plenty for you to chew on and digest for a bit. But still keep on it, that little 1.5 may run high revs in a 4 spd but still got plenty of MPG doing it. The 5 spd was a good addition later on dropping that whine down to a reasonable level at 60 MPH.
Welcome and enjoy.
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#2
by
Puddleglum
on 20 Aug, 2022 11:15
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Thanks for the feedback! I will hopefully be able to do the compression check middle of next week. Sounds like acquiring a dial gauge adapter has got to go on my to do list sooner rather than later. Any recommendations on where to get it? I’m on a tight budget at the moment and have access to standard dial gauges at the hangar. I debated trying to make one on the lathe initially but figured it was more trouble than it was worth. I imagine the cheaper ones out there do function.
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#3
by
Puddleglum
on 24 Aug, 2022 23:58
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Current update . . .
Still waiting on compression tester so no news there yet.
Fabricated timing adapter today. Was at .042” so bumped up to .045” to match Bentley no yellow dot spec. (Mine hasn’t got the dot, but it looks very clean compared to everything else so any paint dot could have been removed when work was done)
I noted that the fuel delivery screw has been messed with. It had been “peppier” than I expected so I turned it down approximately a turn (after experimenting with a few settings) and bumped the idle back up to match. At one stage in the adjustments, with the old fuel and timing setting and about 1000rpm idle, it refused to return to idle if given gas. In neutral with the throttle lever on the pump solidly on its stop, it would hold whatever was the highest rpm reached since disturbed unless drag was added to pull it back down with the tranny. Interesting. It has quit those antics with the current settings.
I got a good junkyard run and got a spare pump for $12 I may try on and compare. From the same rig, I got a clear fuel line which I’ve installed. No bubbles noted when running.
In its present state, the idle is just a wee bit better when warm though I still need to road test the last round of adjustments.
A few more thoughts I’ve been chewing on. The passenger side motor mount didn’t go on so well and is about 3/32 aft of centered in its ring. The motor is aligned so there is no tension between mounts, but to do so I had to leave it imperceptibly rotated counterclockwise on its vertical axis. That uses the full range of adjustment on the front three mounts to relieve the stress. Maybe that’s enough to make it unhappy? Suspect compression is more likely and will update with results.
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#4
by
Puddleglum
on 28 Aug, 2022 22:25
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Well, compression results are in.
#1=355psi
#2=340psi
#3=390psi
#4=395psi
#1 & #2 shot up to 460psi and 510psi respectively with a teaspoon of oil. I didn’t have time to recheck 3 & 4 with oil. This would suggest bad rings as culprit to me. At the moment, I’m thinking I’ll set the car aside till I have the time to put in new rings (hopefully next month). If that fixes the issue, I’ll be on the hunt for a 5spd tranny to hopefully make this a practical car for a longer highway commute. Any tips on reringing before I send Rock auto another order? Is there more to it the meets the eye? I’d be doing it in a gravel driveway but with good access to tools.
When I pulled the injectors this time, there was a significant pool of blackened diesel sitting in three of my heat shields. Just curious if this is something I should be investigating or if it would be normal.
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#5
by
fatmobile
on 28 Aug, 2022 22:55
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There shouldn't be fuel/oil in the injector heat shields.
That's quite the spread on compression.
I usually consider 150psi to be the lower limit.
One the things that can put the kibosh on a rering is the ring gap on the pistons.
How much space is between the rings and piston lands is often too much for a rering.
Where are you at?
I have a spare FF trany I'd sell.
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#6
by
ORCoaster
on 28 Aug, 2022 23:21
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Fatmobile, you can get a diesel in cold weather to fire up with 150 psi in the cylinder? He has a 1.5 liter that he is dealing with.
I would think his two 395s are nearly spec but those mid-300s are not good. The increase, big increase no less with the addition of oil tells me he is due for rings.
Sounds like a winter-time fix-it project to me.
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#7
by
fatmobile
on 29 Aug, 2022 23:51
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Oh did I say 150psi?
I meant 350psi is minimum and probably won't start in the winter.
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#8
by
Puddleglum
on 30 Aug, 2022 22:29
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Fatmobile, I’m in northern Idaho near sandpoint. Whereabouts are you?
This car was horrible to cold start in the winter when I bought it. I figured I was making progress with all the work done, probably just warm temps making it start easy.
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#9
by
ORCoaster
on 31 Aug, 2022 00:03
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Fatmobile thanks for the clarification. I was really wondering about that 150 number. So many of the posts we see had much higher values in the past and yet their complaint was I can't get this sucker to fire up!
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#10
by
fatmobile
on 01 Sep, 2022 23:18
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I'm in Iowa so it gets cold-tested here.
Thanks for pointing out my mistake.
Wouldn't want anyone to think it will do anything but turn over easy at 150psi.
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#11
by
rabbitman
on 07 Oct, 2022 12:14
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Last I checked I had 3 cylinders around 390 and 1 at 420. I can start at about 5F without plugging in the engine heaters, colder than that it cranks too slow.
If it has sat a lot maybe driving it will help the rings out.
My uncle told me that some 1.5 rabbits came with a 4 speed that was geared higher than the 5 speed transmissions. Codes GP, GL and GY have a 0.70 4th gear while a fast geared 5 speed has 0.71 5th gear.
https://www.4crawler.com/Diesel/VW_020_transmission.shtml#4-Speed