I’m looking for technical help or recommendations on: Possible causes for engine popping, and what else I can check.
Hi all. Since a post I made a while back documenting everything from bad compression test results to an engine swap and issues along the way, I hope it’s ok that I’ve made a new Topic fresh with the one issue I have remaining, which should be easier for any helpers to navigate as the other was rammed with info and over 50 posts.
I’m getting an engine popping sound from idol and up through the revs, with the popping getting faster with the revs. There is no major misfire, and car seems to pull well, but the engine will judder every few seconds at idle. Here is a link to the noise being amplified through the air intake - https://youtu.be/376r1z7AxT0
Suspecting possible vacuum leak, I put my original '93 inlet manifold on to the newly swapped '97 engine, where there is now no potential ‘EGR blanked’ issue. I’ve put my original injectors back in, with new seals all round.
I’ve put in new spark plugs and new HT leads. I’ve tested the coil pack for ohms between each lead port (getting good results), but that is the only test on coil pack I have carried out.
I havent yet put my original throttle body on the new engine, however everything else (bar the engine) is how it was before the swap, where no popping noise was evident. The o2 sensor cable was on the exhaust manifold causing the sheath, which is used to keep the four wires together, to melt, but the wire insulation itself looks fine. Unplugging the sensor for a minute changed nothing, but I’m not sure if that’s normal or not. Compression test results were all good, at 180 each, but could it still be a leaky valve which isn’t being picked up?
I’m not sure what else I can check, and I just wanna get back on the road! Any help is greatly appreciated, and again I hope it’s ok that I’ve created a new post, but will add this to the old if it’s a problem. Cheers.
Never heard that noise before. Rather than a leaky valve could it be a sticking inlet valve allowing pressure pulses into the inlet manifold? Try fitting a vacuum gauge to any rubber hose coming from your inlet manifold and see if the vacuum fluctuates.
I have a similar, hiss/pop with my motorbike. It only occurs after initial start and on idle. Like you I checked for leaks on the exhaust and inlet, and double checked spark plugs and even replaced a coil but still does it. I too was thinking it might be a sticky valve, but as it doesn’t give me any problems driving I’m just leaving it! Other than taking it all apart not sure how you can determine if it is a sticky valve, especially as it is intermittent?
Or a weak/broken valve spring?
If so with the cam cover off it might be possible to push down the offender more easily than the others, just by finger tips.
Also it will sound different from the others with the mechanics screwdriver/stethoscope, if you can get it to bear on the head metal next to the offending valve.
Many years ago at Uni, we had one on a nearly new Austin Mini that refused to rev. With just two screws and a reusable thick cork rocker cover gasket we you could see the problems in seconds! Fortunately that car was a doddle to work on if you had supple fingers and wrists. With care, cunning, some improvised tools and four hands, we replaced the spring without taking the head off or losing any bits inside.
Thanks for the replies. I’ll order a cam cover gasket and have a peer in and see what I can see. Might try a set of NGK plugs and leads just to be sure the ones I bought weren’t duff. They were the Bosch Super 4 plugs, which I’ve heard a few people having issues with.
I just took it for a spin and it wasn’t smoking at all, seemed to pull really well again, with just the popping noise getting a little bit louder when the engine warmed up.
And then it’s just hoping it passes the MOT, and maybe driving it will help the issue as it has certainly been sat up for a while. I’ll keep you updated.
Hi again. Just learned that the older Mk1s have 8.8:1 compression ratio and the newer have 9:1 due to domed pistons… I could be entirely wrong here due to limited knowledge on the subject, but could the ECU be chucking in incorrect mixtures of fuel for the ‘new’ engine as car is '93 and engine '97? Not sure how much difference it could make though.
https://www.automobile-catalog.com/model/eunos/roadster_na.html
Bit of an info bank if you are:
1)Stuck with snow.
2)Tier 4 prisoner.
3) Bored.
4) Suffering from Prime & Netflix syndrome.
5) Run out of beer.
6) Waiting for beer delivery.
7) Working from home but you just cannot be motivated.
Cheers for that, makes for some interesting reading. Sadly doesnt give the 8.8:1 ratio I have read about due to domed pistons. But realistically, I’m not gonna take the engine apart any time soon so I’ll just keep driving it 'til I cant any longer…!
You don’t say whether you fitted a new cam belt on your replacement engine. If you have it would be worth checking the valve timing is spot on. If you haven’t replaced the cam belt and if you don’t know the engine history I would replace it and again make sure again that the valve timing is spot on.
I haven’t done yet, I’ve been working on it so much recently I was gonna save this til it gets a bit warmer. But yeh, that will be a job along with the water pump. Maybe it’ll be best to do it sooner rather than later…
Hi again. I’ve not had a lot of time to look in to this issue recently, but I did check the ignition timing. With all the steps in place (linking the jumper cable etc) I discovered that my timing was way out! To the tune of over 20 degrees advanced… However, on setting the ignition timing and base idle the problem was still there, but I discovered something else. I was using a timing light with the spring that you’re meant to connect between the HT lead and Spark plug, but it wouldn’t fit down the plug well so I attached it on the coil pack end. I noticed, when looking at the light flashes, that they weren’t consistent, as in it wouldn’t flash at every revolution of the pulley. It seemed to miss a beat every now and then, along with the slight judder of the engine. Could this potentially be a coil pack issue? As if the light wasnt flashing consistently, you would also imagine that, therefore, it isnt sparking consistently.
Back again, and still at it. Potential Idle Air Control Valve issue? In the linked video, you can here the differences between me unplugging and plugging in the IACV plug. The revs are dropping when removing, which would be normal, however it’s the higher pitched ‘droning’ sound which accompanies the drumming sound. This higher pitched drone sound will go when I unplug, sounding normal (bar the revs drop), and come back when I plug it back in. Any ideas? Cheers
Yes that’s how the IACV works, it’s not on/off it gets sent a pulse at certain frequencies, as the frequency varies the amount of air let through changes as the valve moves further away from it’s resting position. Usually not as noticeable as this to be fair.
edit:
I’ve just read the original issue, when you did the engine swap did you check the IACV wiring was the same? In my old mk1 I swapped out a mk1 1.6 for a mk2 RS 1.8 and the wiring was different for the IACV between the two cars. It would run but suffered from idle droop and misbehaved in cold weather, until I re-wired the plug.
Thanks for the reply. Is there a chance that it could be a sticky valve/sending the wrong pulse, and allowing too much air in? That’s my thinking, which in turn could be causing the slightly rough idle and popping noise, as if it’s a vacuum leak…
just updated my answer with a link, one other thing that’s common is the mounting bolt length varied between different engines and sometimes the gasket between throttle body and manifold doesn’t sit properly introducing an air leak. It also acts like a reed in a flute and make one heck of a noise
I don’t know if the frequency varied between the mk1 1.6 and 1.8 but I doubt it as this was a very common conversion. You can take the IACV valve off and watch it move by plugging the wires in / out.
Don’t expect it to move much, from memory it’s only about 1-2mm
I kept the original intake system, including throttle body etc, from the old engine and changed that over too, so literally the only thing that changed was the block. I may take it apart and have a look though. I had the electrical ‘buzz’ from it, but I’ve heard that’s normal? I appreciate the replies thanks.
I’m afraid not, but I soon got used to it. After having absolutely no adverse effects, even after long drives on the hottest days of summer, I feel like it’s no ‘biggie’, and I have my trust back in the car which I felt was a big thing.
I’m still suspecting the idle air control valve, as, from what I can tell, the noise seems to go away when I disconnect the plug to it, but the car runs in a different manner altogether when I do this so hard to pinpoint. I just haven’t got round to changing the IACV or removing and cleaning. Maybe when it gets a bit warmer, however it runs fine to all intents and purposes.
