misfire just on part throttle
Discussion
This is now driving me mad...
I've just had my 1.8 K series engine rebuilt by a well respected K engine builder, but upon starting it up I've got a mis between 1000 - ~2500 rpm. This makes the engine shake as if it's out of balance fairly noticably, which given that Steve Smith @ Vibration Free has balanced the bottom end is slightly strange. The engine makes full power on the dyno, with the mis not noticable.
The car has an Emerald K3 ECU fitted, and was mapped by Dave Walker on Emerald's rolling road in April this year (first real engine running time since the over winter rebuild). He spotted the misfire, whilst mapping the low load/low engine speed sites.
We swapped the plugs out (old ones looked OK when they were removed).
Since then I've done the following:
- new dizzy cap and rotor arm
- swapped out the HT leads
- replacement coil
I've looked on the live mapping part of the ECU software and can see that the TPS, coolant temp sender and crank speed sensor are all sending sensible outputs to the ECU, so am beginning to run out of ideas.
Historically there have been issues around interference between the king lead and crank sensor signal on K series engines. These wires are well shielded from each other in my installation.
If I hold the engine at (say) 1700 rpm, the exhaust pops and bangs a lot. I did a plug chop last night after holding the car at these revs for about 10 secs and noticed that the #3 plug was carbon fouled.
Given that I've just had the head rebuilt and the clearances and timing, etc will have been set by the guy building the engine, should I suspect a sticking exhaust valve, or could a leak in the exhaust manifold gasket cause these problems?
My next port of call is to take the car to someone with a digital oscilloscope to see if there are any dead spots in any sensors, or signals are breaking down.
I can't see any obvious souce of an inlet air leak where the TBs mount onto the head, but in anycase, this surely wouldn't cause the plug to foul?
Prior to the rebuild/engine removal, the engine was running with no adverse symptoms.
PS any recommendations for anywhere in the London & immediate surrounding areas who have the *diagnostic* skills to determine the cause (i.e. isn't going to just try a load of different parts), who may be able to help?
I've just had my 1.8 K series engine rebuilt by a well respected K engine builder, but upon starting it up I've got a mis between 1000 - ~2500 rpm. This makes the engine shake as if it's out of balance fairly noticably, which given that Steve Smith @ Vibration Free has balanced the bottom end is slightly strange. The engine makes full power on the dyno, with the mis not noticable.
The car has an Emerald K3 ECU fitted, and was mapped by Dave Walker on Emerald's rolling road in April this year (first real engine running time since the over winter rebuild). He spotted the misfire, whilst mapping the low load/low engine speed sites.
We swapped the plugs out (old ones looked OK when they were removed).
Since then I've done the following:
- new dizzy cap and rotor arm
- swapped out the HT leads
- replacement coil
I've looked on the live mapping part of the ECU software and can see that the TPS, coolant temp sender and crank speed sensor are all sending sensible outputs to the ECU, so am beginning to run out of ideas.
Historically there have been issues around interference between the king lead and crank sensor signal on K series engines. These wires are well shielded from each other in my installation.
If I hold the engine at (say) 1700 rpm, the exhaust pops and bangs a lot. I did a plug chop last night after holding the car at these revs for about 10 secs and noticed that the #3 plug was carbon fouled.
Given that I've just had the head rebuilt and the clearances and timing, etc will have been set by the guy building the engine, should I suspect a sticking exhaust valve, or could a leak in the exhaust manifold gasket cause these problems?
My next port of call is to take the car to someone with a digital oscilloscope to see if there are any dead spots in any sensors, or signals are breaking down.
I can't see any obvious souce of an inlet air leak where the TBs mount onto the head, but in anycase, this surely wouldn't cause the plug to foul?
Prior to the rebuild/engine removal, the engine was running with no adverse symptoms.
- any* ideas gratefully received!
PS any recommendations for anywhere in the London & immediate surrounding areas who have the *diagnostic* skills to determine the cause (i.e. isn't going to just try a load of different parts), who may be able to help?
fatjon said:
The data logs from the K3 will tell you if any sensors are giving problems without using a scope. May be worth swapping the injectors around and seeing of the miss moves with them. Have you got it down to a single cylinder or is it more random?
The injecotrs are fired in group mode, so any dodgy feed to a specific injector won't show up in the log. All sensors appear to be OK, albeit with a small amount of noise on the TPS. As belwo, the #3 plug looks a tad carbon fouled, which suggests either an overly rich mixture or a defective spark to that cylinder - however, I've fitted new plugs, new HT leads and a new dizzy cap, and a single coil fires all plugs, so I can't fathom how a single plug is suffering?Will try moving the injectors as you've suggested though.
What you need to achieve is a fair diagnosis of if the problem is external or internal to the engine and swap around of injectors is a good cost free way to go after you are confident of the ignition system. If Dave Walker did the mapping you can be pretty sure no fundamantal mistakes were made, he's up there with the best. A couple of pointers I would mention are that taking the vacuum for the ecu of all runners rather than one gives a better signal at low RPMs, assuming you use vacuum rather that pure TPS. Also putting in a little less ignition advance smooths out the low rpm area with very short bodies where reversion can cause lumpy running and transmission shunting on low rpm part throttle running. Dave did my Cerbera which suffered badly from that problem. He mapped it for best output at low rpm but that was not ideal and a little retard made it much smoother. Once you rule out injectors and ignition it's time for more in depth investigation of the mechanics. I would start with a wet and dry compression test. Air leaks tend to affect one cylinder on a throttle body setup BUT if you are using a lambda sensor and the ECU is set for EGO correction the one pot lobbing air down the exhaust causes the ECU to enrich and then the mixture goes to cock on all the cylinders.
I had a similar symptom on my XE. Misfired up to about 2500rpm and quarter throttle. Turned out to be an injector that wasn't always closing properly. Diagnosed by swapping injectors round and checking plugs. Proof was plumbing the dodgy injector directly onto supply pipe and seeing the fuel dribbling out of the supposedly closed injector. Strangely, I wasn't getting a strong smell of petrol, but the pump only runs for a couple of seconds unless the engine's running. Anyway, one new injector later and it runs sweet as the proverbial nut.
cmsapms said:
I had a similar symptom on my XE. Misfired up to about 2500rpm and quarter throttle. Turned out to be an injector that wasn't always closing properly. Diagnosed by swapping injectors round and checking plugs. Proof was plumbing the dodgy injector directly onto supply pipe and seeing the fuel dribbling out of the supposedly closed injector. Strangely, I wasn't getting a strong smell of petrol, but the pump only runs for a couple of seconds unless the engine's running. Anyway, one new injector later and it runs sweet as the proverbial nut.
Sounds like a very similar situation. I'll get them ANSU cleaned and the spray pattern and pintles checked/serviced. At least I know they're all then providing the same fuel flow.May be totally unrelated but I have a very similar problem on a Puma 1.7 on a part throttle. At around 2000 rpm there's an unevenness where you feel yourself pulled forward and pushed back by the misfire. It's had a top end rebuild by a specialist since it started and a full service but it's still there. Seems fine on WOT and nothing from any of the diagnostics.
Would love to solve it as it's a great car otherwise.
Would love to solve it as it's a great car otherwise.
Problem solved.
The solenoid in the injector was duff at certain pulse widths. At idle it was OK, and at higher duty cycles, i.e. more current, was also OK. On an ANSU machine, the spray pattern, etc was OK. Even when on the 9 cycle program found on most machines, designed to simulate various rpm, the mains current applied didn't replicate the problem.
Using an interupter box (plugs between the OEM injector harness and the injector itself), which showed the pulse width, you could see the current going to the injector was OK, and when increasing the fuelling (group firing mode) the mis went away until the fuel increased to the point where the other 3 cylinders were too rich.
NB this is the first case of this Dave Walker (Emerald) has seen in 15 years. A swap out with another (good) injector, solved the problem....
The solenoid in the injector was duff at certain pulse widths. At idle it was OK, and at higher duty cycles, i.e. more current, was also OK. On an ANSU machine, the spray pattern, etc was OK. Even when on the 9 cycle program found on most machines, designed to simulate various rpm, the mains current applied didn't replicate the problem.
Using an interupter box (plugs between the OEM injector harness and the injector itself), which showed the pulse width, you could see the current going to the injector was OK, and when increasing the fuelling (group firing mode) the mis went away until the fuel increased to the point where the other 3 cylinders were too rich.
NB this is the first case of this Dave Walker (Emerald) has seen in 15 years. A swap out with another (good) injector, solved the problem....
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff