Dead Cylinder / Fuel Injector Question

[peterpilot9]

I have a 1990 NA 278K miles with about 65K on a rebuilt engine and transmission. New fuel pump about 5K ago.

I have a dead cylinder #6 (the balance tube baby). Verified via coil disconnect. I have read through the sticky on the subject but haven't seen my scenario. It seems to be a bit unique.

Engine cranks easily. Idles and accelerates with a bit of vibration but other than that, no problems. No codes or CEL.

1. Verified the coil pack through a swap with a working cylinder. Verified plugs with another swap and installed new.

2. Verified 12V at the coil pack connector.

3. Removed and tested the TPU; within specs. Cleaned the harness connections and ensured no broken/worn wires.

4. Tested compression and all cylinders, including #6 are within 5PSI of each other.

5. Tested the injector and resistance is spec. Using my mechanic's stethoscope, verified it is firing. Sounds identical to the operational cylinder injectors.

So, can't figure out what's up but I tend towards an injector issue. The injectors are actually somewhat newer than the rebuild. No idea why, but I didn't replace them with the rebuild. So, it was a plenum pull and replacement probably about 15K ago.

While the injector on the dead cylinder is firing and is within test specs for resistance, the plug for that cylinder is not fouled. In fact, it looks pretty much like the rest of them. Dry and tan in color. No oil or fuel fouling. No smell of fuel. The car does not smoke at all. Seems like if the injector is putting fuel in the cylinder and there's no spark, then it should be wet. Has anyone experienced an injector that is in spec and fires but is clogged or something like that?

I have seen references to Transistor Power Units that meet specs but still fail to fire a coil. Does this seem to be a valid consideration to you guys?

So any ideas? Thanks for the help.

[Destrto]

I am having a similar issue as you are, currently with my 90 NA.

Just a couple things I have come across during my testing. Have you checked for any vacuum leaks or exhaust leaks? I was able to find an exhaust leak on mine from the EGR ports on the plenum. It never had an issue during the summer, when everything was always warm. Idled fine, never heard a miss. But now that it's gotten cold and all the old gaskets have shrunken up, the EGR plenum gaskets are leaking. Now, I'm still only assuming that's what my misfiring is being caused by, because I've done all the same testing as you have and everything came back within spec.

[peterpilot9]

I am having a similar issue as you are, currently with my 90 NA.

Just a couple things I have come across during my testing. Have you checked for any vacuum leaks or exhaust leaks? I was able to find an exhaust leak on mine from the EGR ports on the plenum. It never had an issue during the summer, when everything was always warm. Idled fine, never heard a miss. But now that it's gotten cold and all the old gaskets have shrunken up, the EGR plenum gaskets are leaking. Now, I'm still only assuming that's what my misfiring is being caused by, because I've done all the same testing as you have and everything came back within spec.

Thanks for the reply and info.

I've traced all the hoses and connections and don't believe I have any vacuum or exhaust leaks. I am not sure how such a leak would leave one and only one cylinder dead. I think leaks such as those would manifest in general engine performance and not just in one cylinder. I could be very wrong on that count of course.

I still tend towards a clogged injector and would like to test it in place but I can't find any kind of connection to the fuel rail itself to test the injector for a pressure drop. Does anyone know if this can be done by inserting a fuel pressure gauge in the fuel line on top of the plenum? I've seen multiple examples of these gauges being inserted in the line just past the fuel filter but don't know if this location would suffice to catch the pressure drop when activating the injector. Seems like it should so if anyone has done this or has some knowledge on the topic, I would appreciate any advice.

Thanks again for the help.

[Destrto]

I agree, it does sound strange.

I'm not sure of a way to test for a pressure drop on individual injectors with them still installed on the car. The only method I know of is testing flow after they've been removed.

[peterpilot9]

I agree, it does sound strange.

I'm not sure of a way to test for a pressure drop on individual injectors with them still installed on the car. The only method I know of is testing flow after they've been removed.

Well, I was wondering that myself and there is a way to test the injector while in the car. Using a device generically known as an injector pulse tester, you can run an injector with the engine off after the system has been pressurized. I'm satisfied that with the pressure being static, the location of a pressure gauge just after the fuel filter will give an accurate enough result.

I have ordered an EM276 fuel injector tester manufactured by AllSun. I've seen some demos on You Tube and it looks like I can use it on #6 without removing the balance tube.

https://www.ebay.com/itm/Automotive-Fue ... 100623.m-1

So my plan is to get the device, pulse test this injector to see if it is providing fuel or if it's clogged. I may even balance test the remaining injectors to see what kind of condition they're in. Might video the process and post it on YouTube; share the link here in case anyone else wants to see how it works on our cars specifically. I think this is going to be a good tool to have given how difficult it is to remove the injectors on this car. Be able to check their function before the headache of a plenum pull.

If by chance the injector proves to be providing fuel, then I will work on the assumption the problem is in the TPU and/or harness. Will probably be late next week before I get the device so I'll update then.

[Ace2cool]

It's still sounding like a dead injector to me. The ohm test will usually pinpoint your issue, but I've had a dead injector that ohmed out exactly in the middle of spec. Which PTU do you have, silver or shiny black?

[peterpilot9]

It's still sounding like a dead injector to me. The ohm test will usually pinpoint your issue, but I've had a dead injector that ohmed out exactly in the middle of spec. Which PTU do you have, silver or shiny black?

I agree. The thing that throws me off is that you can hear the injector firing (clicking) with the stethoscope when the car is running and it really sounds like all the rest. In any case, the injector pulse test should let me about the function of that injector. Should get the testing device early next week

I have the Type 2 TPU; shiny black not the square silver block.

[amc49]

I assure you that a vacuum leak if located right can kill one cylinder without any apparent affect on the others, it depends on the location, like a single runner well down it, the backflow to get to other cylinders being enough that time affects the others not getting the leak affect. Haven't seen it specifically on these but on other cars yes absolutely. The true sharing of the leak with other cylinders is based on how close to a mixing plenum the leak is. Very possible on two piece intakes where the join is in pure runners only, one runner faults a common gasket right in that one runner and there you have it. The others may never see it.

As well, the overall compression not given other than approved of and if anywhere down around 100 psi the engine not to mention the cylinder is dead. A whopping lot of people think compression is fine until you nudge them for the numbers and then that all changes. Plug color though says compression is OK.

[peterpilot9]

I assure you that a vacuum leak if located right can kill one cylinder without any apparent affect on the others, it depends on the location, like a single runner well down it, the backflow to get to other cylinders being enough that time affects the others not getting the leak affect. Haven't seen it specifically on these but on other cars yes absolutely. The true sharing of the leak with other cylinders is based on how close to a mixing plenum the leak is. Very possible on two piece intakes where the join is in pure runners only, one runner faults a common gasket right in that one runner and there you have it. The others may never see it.

As well, the overall compression not given other than approved of and if anywhere down around 100 psi the engine not to mention the cylinder is dead. A whopping lot of people think compression is fine until you nudge them for the numbers and then that all changes. Plug color though says compression is OK.

Thanks for the information regarding the effects of a vacuum leak on a given cylinder's performance. I'll keep looking into that as an issue as I work to verify the operation of the injector.

As for the compression, to offer clarity, the readings are as follows; #1 180, #2 182, #3 178, #4 182, #5 178, #6 180. Taken with a Harbor Freight tool so there is that......

[amc49]

Compression is fine there. Just wishing more who check it would give the numbers right off the bat, it saves extra texting. One would be amazed at how many people think that just putting a finger over a plug hole and feeling anything at all means that is not a problem. Thanks for getting back on that.

[DCaff300ZX]

I assure you that a vacuum leak if located right can kill one cylinder without any apparent affect on the others, it depends on the location, like a single runner well down it, the backflow to get to other cylinders being enough that time affects the others not getting the leak affect. Haven't seen it specifically on these but on other cars yes absolutely. The true sharing of the leak with other cylinders is based on how close to a mixing plenum the leak is. Very possible on two piece intakes where the join is in pure runners only, one runner faults a common gasket right in that one runner and there you have it. The others may never see it.

As well, the overall compression not given other than approved of and if anywhere down around 100 psi the engine not to mention the cylinder is dead. A whopping lot of people think compression is fine until you nudge them for the numbers and then that all changes. Plug color though says compression is OK.

That's great info, never knew about that specific vac leak info so thanks for sharing!

[peterpilot9]

I have completed pulse testing the injectors and the dead cylinder's injector is not firing. Not sure I can explain how it sounds so strong with the stethoscope, but when pulsed with the tester, it is just dead.

Only tested 1, 2, 3 and 6 because of the hassle getting to the connectors but I think it was enough of a comparison to see what the problem is. Except for 6, you could hear them fire and see a pressure drop. Not sure what the standards are but my drop was 12PSI for each cylinder that worked. Again, a Harbor Freight pressure gauge with a bit of a seep that I couldn't correct but given that they all dropped except for the dead one, I feel comfortable I found the problem.

One piece of advice for anyone who is thinking of getting a pulse tester to use on this car. Get a spare injector connector clip with the pigtails and configure it to connect to the test device. Using alligator clips is not an optimal solution. Works, but takes probably twice as long.

So, moving onto a plenum pull and injector replacements. Thanks to everyone for the input and responses. I'll update once I've done the replacement.

[Ace2cool]

Be sure to do all the deletes while you're in there. Especially the water lines.

[peterpilot9]

Be sure to do all the deletes while you're in there. Especially the water lines.

Thanks and I'm sure that's good advice. This is my second plenum pull on the car and I left all intact last time around. Most likely will delete the water lines this go.

[Ace2cool]

Oh wow, you're a trooper then. Plenum pull and replace goes from 4 hour to 45 mins with the deletes. Even faster if you're experienced.

[peterpilot9]

Finished the repair and test drove her this afternoon. I'll post a bit of my experience for those who might be interested.

It was indeed the #6 injector. I just noticed that I posted something incorrect in my original post. I DID NOT get a resistance reading within specs when I originally worked to trouble shoot the dead cylinder issue. I couldn't really get the probes onto the connecting pins and didn't have any luck with the alligator clips. When I pulled the injector, I got somewhere around 70 ohms n the bad injector and about 14 ohms for the remaining injectors.

I completed the coolant line bypass. One thing I noticed was that while the 5/16" heater hose that I bought retail had an ID that fit very well onto the hard pipe connections, the OD was somewhat larger than stock and was a bit more bulky in the confined spaces. I did not complete an EGR delete.

The car is running better than she has in a very long time, especially in terms of the idle. Smooth as silk and perfectly compensates for the (un f'ng believable) cold and ac compressor.

Likely because I pulled the IACV assembly and cleaned the AAC spring and chamber as well as all the associated connectors. A bit of a lime green fungi factory on those pins. In addition, I replaced the rear PCV hose as it broke when I tried to remove it to access the right side plenum coolant line for the bypass. I believe it had been "repaired" at some earlier point as it was completely brittle and wrapped from one end to the other with some kind of black tape. Cleaned up all vacuum point connections as well. Really happy with the result.

Speaking of vacuum, I did have an issue that is not yet resolved. The front passenger side plenum retention bolt (the short one) hole appears to have some threads stripped. I was unable to finger start any of the bolts into that hole. So, I decided in my partial state of hypothermia (did I mention it was cold in my garage?) to go ahead and complete the assembly and deal with it at the end. Well, no bolt and no vacuum leak that I can detect. I will resolve this issue shortly but was quite surprised that it didn't cause some operational issues.

Last item is that during the test drive, the heater provided hot air but the temp gauge never moved off the bottom peg. I pulled the gauge sensor and gave it a good cleaning as it too had grown a fair amount of the green stuff. If that doesn't work, I'll get to work fixing that issue.

Anyway, thanks to everyone for their help and contributions. I've had this car for 18 years and about 190K of my own miles so I guess I am somewhat attached. It is a daily driver, not pampered in any way and has overall proven to be a very reliable transport. And I still love the way she looks and drives so I'll keep putting parts on as the old ones wear away.

[DCaff300ZX]

Awesome results, definitely keep hunting the various connectors and harnesses for that green crud- GUARANTEED it's all over given the non-shielded connectors Nissan used, and which causes many issues. If you haven't checked the PTU harness connectors do so, that may also calm a few things down...CAS can also be corroded and an issue same as your CTS you cleaned, and the injector harness, and IACV. A new upgraded IACV harness is a great idea as it's shielded, as generally those connectors are a cracked-up, broken, corroded mess.
When you get a chance I definitely also put that bolt in whenever you can- a larger size and corresponding thread tap, trying to vacuum any chips.
Good Luck, and keep on keepin' on with your Z!

[SEWashington300ZX]

peterpilot9 - did you, or had you, upgraded to the new style of fuel injectors? I need to replace them on my 1990 NA and am looking for informed opinions.

Much thanks.

[peterpilot9]

Apologize for the very late response. My car has not been upgraded to the new style injectors so I can't be of any help there. Sorry.