Hesitant / Stmbles on quick but not slow acceleration

SpudgunBlunder · Post by **SpudgunBlunder** » January 6th, 2007, 7:14 pm

My MX-3 V6 24v 1845cc (K8) 1995 has been running ok until I did a compression test the other day. Went to drive to work and tried to pull away into the main road and it nearly stalled on me!

SYMPTOM:
Car starts fine and idles fine and will even rev high if reved very SLOWLY. Even whilst driving on the motorway I can do 80 - 90 or whatever, it just takes time.
But if I open the throttle quickly it stutters and revs drop straight away and will not increase if the trottle is held open.

PROBABLE CAUSE:
I did a compression test because the engine was overheating all of a sudden. Could'nt find any leaks and it kept filling the overflow tank. Friend told me it could be a bad Head Gasket.
However I did the test wrong.
I took one spark plug out at a time,
Screwed in the pressure gauge,
Unplugged the plug lead from the dispributer (from the plug i'd taken out),
then RUN the engine for a few seconds - reving a few times, noticing the different readings.

Also made the mistake of leaving off the Resonance Duct when testing chamber 3.

TESTED:
(Following Troubleshooting from Manual F2-048)
All plugs were in ok condition when doing compression test:

I did test the sparks on the inlet manifold during the compression test but just got dull yellow sparks.

I've tested the PD between the battery GND and engine (0.3v)

Plug leads are about 20% higher resistance than spec. I think chamber 1 was 21k ohm.

Took off distributer cap. Terminals were covered with white oxide (I think). So dremeled them clean and shiny.
Primary and secondary coils are in spec. Primary was 1.2 ohm but then measured resistance of probes which were 0.5 ohm. --> 0.7 ohm

VAF resitance open and closed is in spec.
(However - NOT LINEAR - is this OK or NOT OK??? Read a post about this but still not sure)

Checked for vacuum leaks. None - I think but not had manifold off.

Taken off throttle body which is covered in oil and inside manifold.
Adjusted throttle position sensor from 0.63mm on/off to 0.5mm as spec.

VRIS solenoid next to air filter is working and vacuum is present. When open throttle quickly vac goes but when I open throttle slowly vac stays.

Have not got any error codes from ECM but don't know yet if there are none to show or if I just can't get them.

Can't test manifold pressure but when throttle is opened quickly there is very high vacuum then nothing.

Not checked ECM yet (2F, 2I). Thought it was much harder to get at than it is.

Can't check fuel line pressure but fuel pump under back seat is working. Took out while engine was running. (bit dodgy

)

Can't test timing advance properly but will slightly adjust distributer.

Can't think of anything else other than previous owner (friend) had symptom before but took it for a spin and it went away.

Please help. I'm desperate for my car back.

Bumpysbro · Post by **Bumpysbro** » January 6th, 2007, 7:50 pm

when did you last replace your plug wires, get a ohm reading on them. i had just one of them bad it would start and drive but when you try to floor it quickly it would stumble like you said.

SpudgunBlunder · Post by **SpudgunBlunder** » January 6th, 2007, 8:15 pm

could one or two bad (high resistance) leads have this much effect?

I did test them and there were none over 21k ohm.

What would you call a bad lead? Manual says 16 k ohm per meter but is it 16.1k per meter or 16.1G ohm?

I will try to get a new set. Anywhere in uk to get parts cheap?

SpudgunBlunder · Post by **SpudgunBlunder** » January 6th, 2007, 8:27 pm

Should the PRC Solenoid Valve cut the vacuum to the Fuel Pressure regulator when you accel?

When I accel slow the preesure remains but when I accel fast the vacuum drops. But is this due to the drop in engine speed?

Bumpysbro · Post by **Bumpysbro** » January 6th, 2007, 8:50 pm

well the more power and the more you accelerate. the less vacuum there will be. when at idle you have the most vacuum. ya even one can make it stumble. if it is bad basically when you floor it, more air more gas but the spark doesnt increase which means you got a bad burn and it will run like crap or even stall. but same thing with fuel pressure. have you did a vacuum test on the engine? a tester would be the best, see if theres a leak somewhere. you know something to check on the v6 do you have a vacuum chamber? should be connected with the intake and a vacuum hose somewhere. i had gas in mine, and it killed my car made it stall cuz the vacuum was cut off. mine were about 8.2k each but one was way off so i switched them out and bam.

either its a bad burn, vacuum leak or fuel related. since the tps was checked and reset correct?

you know if your throttle body is covered in oil i would think you would have a pcv valve malfunction. since that vents gases from the crankcase to the intake.

best bet is to rent or buy a vacuum tester. itll tell you alot, valves not sealing or a leak somewhere.

SpudgunBlunder · Post by **SpudgunBlunder** » January 6th, 2007, 9:59 pm

I tested the PCV valve according to F2-124 in the manual. It's ok. I always thought it sounded faulty as it ticks rapidly?
But I've never come accross one before so I don't know what it should sound like. It's covered in oil tho.
But I've only had it for less than a month.

so I don't know any history really.
Could the oily intake be due to the EGR? Although there is dirt and oil on the OUTSIDE of the throttle. The EGR recirculates exhaust after the throttle according to F2-125.

you know something to check on the v6 do you have a vacuum chamber? should be connected with the intake and a vacuum hose somewhere. i had gas in mine, and it killed my car made it stall cuz the vacuum was cut off.

Do you mean the 6mm diameter hose from the fuel pressure regulator to the PRC valve? If I block this with my finger it does'nt seem to make a difference.

SpudgunBlunder · Post by **SpudgunBlunder** » January 6th, 2007, 10:06 pm

Oh yeah forgot to mention that after the dodgy compression test my plugs are black. So lack of air? or I read on here that if the ECM thinks something is wrong it runs the engine rich to prevent knocking.

Know where I can get some HT leads from in UK near Cheshire if pos?
(Just sold 6 month old distributer and new leads for £60 on eBay

)

Bumpysbro · Post by **Bumpysbro** » January 6th, 2007, 10:18 pm

lack of air=vacuum leak most likely egr will cause a huge vacuum leak if its faulty and for the other it could be your fuel regulator, because if you disconnect the hose it should raise the fuel pressure and see a difference in idle.

well oil getting into the intake is either caused by leaking valves or the pcv is bad.

If EGR is on during idle, stumble and even stalling will likely result. If EGR is on during hard acceleration, low power (from reduced air/fuel volume) is the result. At part load, lean mixtures for economy create high combustion chamber temperatures and without EGR, these conditions not only create Nox, they also foster pre-ignition.

could be a clogged or choked EGR valve, a ruptured EGR valve diaphragm, split or improperly routed vacuum hoses, or other component failures.

ariesdude · Post by **ariesdude** » January 7th, 2007, 4:00 am

Sounds like a vaccum leak - the weakest part is the air hose from the filter box to TB (the accordian like part) - try stretching it and look at it from all angles. If there is a minute crack it will operate normally under slow acceleration - but under fast acceleration (lot of air being sucked in) the crack would widen and let in unmetered air causing the engine to stumble or stall.
If that hose is ok then try replacing the cat with a test pipe and see if problem still exists.

SpudgunBlunder · Post by **SpudgunBlunder** » January 7th, 2007, 7:42 am

I've checked the inlet pipes. Look fine. Can't hear any whistling either.

Will check the EGR today but the manual does'nt mention it for this problem?

What would have starting the engine without the Resonance Duct have done? Could it have messed with the ECM or the spark advance timing? I thought I may have bust the VAF but I've swopped it with another and no change. I also tesed both. Both are in spec and both don't behave linearly. Are they both knackered?

I'll try and post a video on Youtube or something. Might help.

Cheers guys+girls

SpudgunBlunder · Post by **SpudgunBlunder** » January 7th, 2007, 2:03 pm

I've taken a video with my girls V3 Razor so not the best quality but you can see exactly what I mean. Also a vid of the exhaust. Can hear a few back fires/misses.

Hope this helps:

http://www.youtube.com/watch?v=m6Nq3GJ-tMA

http://www.youtube.com/watch?v=L6kqx6ZYGs8

ariesdude · Post by **ariesdude** » January 7th, 2007, 2:53 pm

One way to check for vaccum leaks is to hold the rpms steady and spray propane gas (or wd-40) around the air tubes - if the rpms go up then you have a leak.
Other possible options are a clogged fuel filter, clogged catalytic converter, faulty ignition module(distributor) etc.

SpudgunBlunder · Post by **SpudgunBlunder** » January 7th, 2007, 4:38 pm

Thanks for the suggestion. I'll try the wd40 in a sec. My friend thinks it will be the distributer. He thinks I damaged it somehow doing the compression test wrong.
I've checked both coils and they're in spec so could it be the distributer cap? Anyone know how much this will be in UK?

SpudgunBlunder · Post by **SpudgunBlunder** » January 8th, 2007, 7:50 am

WD40 trick did'nt show up anything.

Could the timing advance be wrong? I can't measure this though. Should I adjust the distributer slightly and try it? (whilst jumping TEN & GND)

SpudgunBlunder · Post by **SpudgunBlunder** » January 8th, 2007, 2:59 pm

I replaced the spark plugs and it's made a big difference. Not so hesitant now but it's still there (driveable just about). It's back firing alot though.

I'm trying to get a 2nd hand distributor cap and leads to try that. But I need to make sure the cap is the same as the ford probe and mazda 626. Not advertised with year. It looks the same.