Mi16 engine not starting after rebuild

[davemar 16]

I've just rebuilt mine Mi16 engine (XU9J4) by using a bottom end (including block) I had kept spare (sat in my garage for years) underneath the head from the engine that I've been using in the car. So I knew the head is fine, as it was working ok, and just needed cleaning up. The bottom end had new bearings in it and the crank was good. All I did was to dismantle it to check nothing had seized up and give it clean up. I gave the liners a little bit of a de-glazing to clear a few tiny rust marks off them. So apart from cleaning everything up I replaced the valve stem oil seals, camshaft seals and head gasket. So everything should be good.

 

I've now refitted everything else, including the electrics, which were working perfectly well before dismantling a few weeks ago. However the thing doesn't want to fire up. The injectors are squirting fuel nicely as I've checked those. I seem to be getting a spark (by checking the spark plugs out of the engine). So I'm a bit stumped. The starter is spinning the engine over nicely. While I think the pistons and liners are sealing OK, could they prevent the engine starting if they had a problem?

 

[petert 586]

Well if you've tested fuel and spark, you've confirmed two out of three. Do a compression test to get three out of three.

[davemar 16]

Yep, good plan. I'll have to hunt down my compression tester buried somewhere in my garage. What sort of figures should I expect?

 

[pug_ham 244 3 Cars]

Simple one, are the HT leads in the correct order / position?

 

Are you using an Mi16 ecu or dizzy adapter?

 

g

[davemar 16]

Yes, HT leads in correct order as they are tricky to get in the wrong order due to their lengths. I'm running with a Mi16 ECU and dizzy cap, so no 8v mash-up.

 

[Richard309Sri XU5JA 205GTi 8]

15 hours ago, davemar said:

What sort of figures should I expect?

10 bar (145PSI ) min - 

 

anything less is on the weak side 

anything above all good. 

[davemar 16]

Here's the results (PSI):

1: 110

2: 85

3: 90

4: 105

So far lower than they should be, but it was on a stone-cold engine.

 

I think my valves are fine (they certainly were before the rebuild), so reckon it might be piston rings / liner end of things.

Another possibility is if the new head gasket I've fitted is problematic. Could it be too thick causing a low compression?

Edited November 3, 2021 by davemar

[Thijs_Rallye 87 4 Cars]

Compression is very low, nonetheless if should be able to at least give some signs of life imho. Cams timed correctly? Coolant temperaturesensor values ok?

[SRDT 175]

You could have a weak spark with spark plugs out of the engine and no spark with the pressure inside the cylinders.

[petert 586]

Should be 200-210 cold, throttle open whilst testing.

[DamirGTI 342]

1 hour ago, SRDT said:

You could have a weak spark with spark plugs out of the engine and no spark with the pressure inside the cylinders.

 

My assumption as well ... spark problem , weak or none at all whilst inside the cylinders  .

 

I had a few engines with such issue ... it bite me in the **** and made me wtf head scratching tearing engine bay apart losing a lot of hours/days .. only to eventually find it's the spark problem (precisely spark plugs , found most often happens when using "fancy" multi electrode plugs .. for some reason certain engines really dislike brand and type specific spark plugs no matter brand new ones or used/old)

 

Unfortunately , not as easy to test either ... as something obviously happens with the spark whilst within the cylinder (pressure , vacuum , fuel mixture etc.) and it simply won't ignite .. not even a cough ... not even on a start spray fluid test , yet there seems to be spark when testing the plug outside off/pulled out from the cylinders .

 

D

 

[davemar 16]

I'll try a fresh set of plugs as I do have some around. I'll also give the dizzy cap contacts a clean up as well. I want to try as many things before committing to dismantling the engine again!

 

 

[SweetBadger 94]

Try putting a tea spoon of oil down each bore and repeating the compression test. If cylinder pressure jumps up significantly then it’s likely you have a ring sealing issue.

[Richard309Sri XU5JA 205GTi 8]

5 hours ago, SweetBadger said:

Try putting a tea spoon of oil down each bore and repeating the compression test. If cylinder pressure jumps up significantly then it’s likely you have a ring sealing issue

yeah think that'll be a a good idea  - aka "wet compression test" - as you are getting "large" variation in compression between adjacent cylinders - more than 10%  

 

2/1 = 85/110 = 77%

3/4 = 90/105 = 86% 

 

should be all similar even if lower, like between 1 and 4 is only 105/110 = 5% difference. 

 

know you said the valves ok but could be worth checking the valve clearances? 

 

did the new spark plugs get it started? 

 

 

[davemar 16]

Just done a wet test and here's the results (I'll list the dry ones next to them):

    Dry    Wet

1: 110   140

2: 95     110

3: 90      60

4: 105   115

 

So quite a lot of variation there, and something weird going in with cylinder 3.

 

I fitted new spark plugs and that made no difference. It certainly sounds like it wants to fire when cranking with a bit of a hiccup rather than just spinning away.

 

I'm tempted to try and blow air into the spark plug holes at TDC to see if there's any leaks, but lacking anything I can put down the hole and seal it in. Maybe I could butcher an old spark plug to make something.

 

[welshpug 1,657]

have you checked the timing pins still go in?

[Richard309Sri XU5JA 205GTi 8]

3 hours ago, davemar said:

I'm tempted to try and blow air into the spark plug holes at TDC to see if there's any leaks, but lacking anything I can put down the hole and seal it in. Maybe I could butcher an old spark plug to make something.

you could get a leak down tester pretty cheaply, might be quicker and easier. 

 

or some people adapt the compression tester hose 

[DamirGTI 342]

4 hours ago, davemar said:

It certainly sounds like it wants to fire when cranking with a bit of a hiccup rather than just spinning away.

 

 

 

If there's at least a sign/feel it want's to fire ("coughing") , try to feed it with some easy start spray .. and see what happens . 

 

Compression is a bit odd across the 4 ... but me thinks so too it should start despite that , running bad first few times and hopefully would then "let lose" and with the compression then getting better after warm up/cool down .

 

Is that the old block with old piston rings and liners , or rebuilt with the new rings (or fitted back old used ones ?) and honed/de-glazed the liners ?

 

D

[Toddy 7]

 

check the cam timing

try a known good coil + ignition amp.

[Richard309Sri XU5JA 205GTi 8]

On 11/4/2021 at 2:58 PM, davemar said:

Just done a wet test and here's the results (I'll list the dry ones next to them):

    Dry    Wet

1: 110   140

2: 95     110

3: 90      60

4: 105   115

 

So quite a lot of variation there, and something weird going in with cylinder 3

could be too basic to be asking but just checking - 

 

were the pistons rings offset 120degrees (on plan in rotation about the centres) -

each one of the three rings per cylinder? so the gap in the rings is not lined up -if you can see what I mean?

 

mainly asking for cylinder 3  in particular 

[petert 586]

The reality is, with numbers like that you have to pull it apart. If it were a cam timing issue, all the numbers would be consistently low (or high). 

[davemar 16]

The rings were all rotated so the gaps were 120 degrees apart.

 

Quick background:

The engine was running fine, but suddenly decided to sound like a bag of spanners a few weeks ago. I suspected either piston rings or a bottom end problem, so stripped it down. It turned out it was just a loose flywheel, so I didn't really have needed to strip the engine. Unfortunately when removing the head one of the bolts snapped making the block unusable unless I could remove the remains, which was too much hassle. So I decided to swap the bottom half for a spare one I already had, and use the existing head. The head gasket also looked liked it was not long for this world, so not a bad decision to remove the head anyway.

 

Basically the spare bottom half engine has been sat around in my garage for several years after I did a rebuild on it. So then it had new bearings, piston rings and replacement crankshaft. As part of the installation this time I decided to strip it down and rebuild it again as it had been sat around for so long without moving, so wanted to check nothing had seriously corroded in that time. Only the liners looked a bit tarnished, the liner seals were rotten so needed replacing, and a general covering of dust which needed cleaning off.

 

So the current engine is with a known good head which I replaced the valve oil seals and cleaned up; and replacement bottom end with 'new' (i.e. unused) bearings and rings. All the ancillaries (manifolds, sensors, wiring, etc) are exactly the same as they were when the engine was running perfectly well a few weeks ago. So I'm pretty confident all those parts are in good working order.

 

My instinct is towards the liners and rings where sitting around getting exposed without moving has affected them more than is acceptable. At least the bearings retained a film of oil on them and kept away from dusty moist air.

[davemar 16]

I double checked the timing alignment today and the crank and cam pulleys were all lined up correctly, so I think that one can be ticked off.

 

[petert 586]

1 hour ago, davemar said:

I double checked the timing alignment today and the crank and cam pulleys were all lined up correctly, so I think that one can be ticked off.

 

Stop dicking around and strip it down. You know it's fµcked.

[davemar 16]

Yep, you're right, I'm going to have to strip it down again.

I did managed to do a smoke test with the piston at TDC and the valves shut. The smoke came out of the oil filler hole, so that suggests the piston rings aren't sealing. So they'll have to come out.