VERY LOW COMPRESSION

[Ihtisham Rauf]

Hi people,

My car is now on bike throttle bodies.
I can't get it to run with the MAF connected.
The MAF is mounted at the inlet of my pipercross airbox.
It runs pretty well with the MAF disconnected.
However, when starting from cold, it struggles to start and then you have to feather the throttle to keep it from stalling. I do this for a few min and then the revs stabilise by themselves. On a warm engine, it is much easier to start, but still needs a little throttle to start.
With the MAF connected, it starts much easier, and the revs stabilise immediately regardless of whether the engine is hot or cold. The problem comes when actually driving the car. Any throttle causes the car to bog down and eventually stall. So I have no option but to drive with the MAF disconnected, with the only downside being difficult starts from cold.

I did a compression test today. I got:
Cyl 1 - 70psi
Cyl 2 - 60 psi
Cyl 3 - 50psi
Cyl 4 - 50psi

These are very low numbers. Is this because, during the compression test, I cranked the car without having my foot on the throttle? Because like I said, the car doesn't switch on anyway if I just turn the key without any throttle. Should I redo the test with a little feathering on the throttle in the same way as I would when actually starting the car?

What I found confusing is that, even with these ridiculously low comp results, the car still drives great and pulls REALLY WELL.

 

[MattInARedMicra]

because you take the spark plugs out to compression test anyway, the car should never be able to run during one

It's measuring compression when the starter motor spins the crank

 

[Ihtisham Rauf]

because you take the spark plugs out to compression test anyway, the car should never be able to run during one

It's measuring compression when the starter motor spins the crank

So something is majorly wrong then?

 

[Ihtisham Rauf]

My bike throttle bodies have no idle air control valve. Could this mean holding throttle open is necessary for my case in order to get decent compression results.

Because otherwise, how is air supposed to get into the engine?

I read this website:

Why Open Throttle for Compression Test?

Compression testing guides tell you to hold the engine’s throttle wide open during a compression test; this post tells you why it may or may not matter.

gtc.ca

 

[.Maarten]

You need to do a compression test with the throttlebody wide open. Else the air cant enter the cilinder and youre having the engine pulling the air tru a small hole. Ergo wont build much compression and will result in low numbers

 

[Ihtisham Rauf]

You need to do a compression test with the throttlebody wide open. Else the air cant enter the cilinder and youre having the engine pulling the air tru a small hole. Ergo wont build much compression and will result in low numbers

Right makes sense.
Will redo it tomorrow with the throttle wide open.
Hopefully we'll get some better results lol!

Thanks as always!

 

[Ihtisham Rauf]

Rather than starting a new thread:

Basically I want to get the car running with the MAF as itll be better for the car.
Currently it wont run with the MAF and I think it is because there is no mesh in front of the MAF to straighten the air flow.

My idea is to use the stock almera MAF housing (which has the mesh) and use a 100mm to 70mm silicon reducer to connect my 100mm airbox inlet to the 70mm maf housing. However, this will mean the maf wiring is not long enough to reach the maf.

To extend the wiring, is it just a case of cutting the plug off, crimping some wire in the middle, and reconnecting?

Thanks for any advice!

 

[.Maarten]

Yes you need the mesh. And you can just extend the wires like that.

 

[Ihtisham Rauf]

Yes you need the mesh. And you can just extend the wires like that.

Great.

Hopefully it will run with the MAF once I do this.

Thanks.

 

[Ihtisham Rauf]

Redid the compression test today while holding throttle wide open. Results were:

Cyl 1 - 125
Cyl 2 - 125
Cyl 3 - 137
Cyl 4 - 125

Do these seem okay given I am running a 774 inlet cam?

 

[frank]

bang on i would say

 

[Low Rider]

They're lower than I'd expect. Done on a hot engine?

My turbo engine with low compression pistons and 288deg cams makes 165psi across all 4 cylinders.

 

[Ihtisham Rauf]

They're lower than I'd expect. Done on a hot engine?

My turbo engine with low compression pistons and 288deg cams makes 165psi across all 4 cylinders.

It was done on a warm-ish engine running ITB's. I don't know if ITB's make a difference to compression though :/

This was with the MAF disconnected as well. Again don't know if this would make any difference.

 

[SuperUno]

Seems ow, I would like to be seeing 150psi+ this could explain your oil consumption....

 

[Ihtisham Rauf]

Seems ow, I would like to be seeing 150psi+ this could explain your oil consumption....

Ah okay yes putting 2 and 2 together.

On the day I did the compression test, one thing to note is my battery went fully flat the day before (constant starting and stalling while trying to sort MAF issues), so I don't know if my battery was close to fully charged on the day of the test, and how much this wouldve changed things.

I am putting a 774 exhaust cam in within a few days and will redo the comp test. Been advised by matt to also do a wet comp test. I've looked online and it seems I am looking for the results to be no more than 10% higher than the dry comp test.

Will see what happens! Appreciate the input man

 

[plmval]

Ah okay yes putting 2 and 2 together.

On the day I did the compression test, one thing to note is my battery went fully flat the day before (constant starting and stalling while trying to sort MAF issues), so I don't know if my battery was close to fully charged on the day of the test, and how much this wouldve changed things.

I am putting a 774 exhaust cam in within a few days and will redo the comp test. Been advised by matt to also do a wet comp test. I've looked online and it seems I am looking for the results to be no more than 10% higher than the dry comp test.

Will see what happens! Appreciate the input man

Did you do the compression tests with the extra-long corrugated hose connected or not & the throttle plates held wide open?

May I suggest to redo the comp tests & get the values more equal & correct before changing cams to enable valid comparison of results & to get the thing running correctly before changing modifying more items?

Cam change may interdict valve clearances & timing introducing additional potential fault issues?

A conservative sequential logical fault finding approach is always quicker, less unnecessary work, cheaper & more rewarding than the scattergun approach changing parts at random?

Good luck

 

[frank]

Ah okay yes putting 2 and 2 together.

On the day I did the compression test, one thing to note is my battery went fully flat the day before (constant starting and stalling while trying to sort MAF issues), so I don't know if my battery was close to fully charged on the day of the test, and how much this wouldve changed things.

I am putting a 774 exhaust cam in within a few days and will redo the comp test. Been advised by matt to also do a wet comp test. I've looked online and it seems I am looking for the results to be no more than 10% higher than the dry comp test.

Will see what happens! Appreciate the input man

the exhaust cam wont make any difference ihtisham, that inlet cam and slack chains will have dropped about 30psi from haynes specs tho

 

[Ihtisham Rauf]

the exhaust cam wont make any difference ihtisham, that inlet cam and slack chains will have dropped about 30psi from haynes specs tho

Yes I just wanted the exhaust cam more just to finish the car off tbh as I probs wont be doing much more with it now.
My power graph shows peak power at 6.7k.
Been advised by a few of the rally people to put the 774 exhaust cam in as well to maybe make a little more at the peak, as 774 cams are supposed to have a power band up to 7.5k, mine drops off at 6.7k,

The exhaust cam is one of them things I've wanted to do for a while, but didn't, due it not making much difference.

I probably wouldn't have bought it but I got lucky in that humphris were selling the pair at a discount £400 rather than the standard price of £500, I'll be selling the inlet cam from the pair, as I already have the 774 inlet installed, so the overall cost to me will hopefully be much less than what a standard 774 exhaust cam is!

And yes plmval, was done with throttle wide open and the hose was just a rubber one that I got in the comp tester kit.

 

[fuzzy-hair-man]

From watching your cam install video you didn't adjust valve clearances when installing the new cams, this would effect valve timing and therefore compression values, it may be the source of variances between cylinders.