Interesting compression test in my old truck

[85 Cutlass Brougham]

Well, I have had a problem with a rough idle and a persistent code P0303 (cylinder 3 misfire) in the OBD II system on my 1998 Nissan Frontier for a while now. So, I decided to run a compression test to see if the P0303 was due to a failing cylinder. The spec for compression in this engine is a max of 178 psi and a minimum of 149 psi according to my FSM. The first test was performed wrong with me doing 2 tests per cylinder which wore down the battery and slowed the engine down by cylinder's 3 and 4, but provided a variance of 189 psi for the highest and 170 psi for the lowest. (I re-ran cyl. 1 and 2 after I finished and it had dropped 10 psi, so I figured I had done it wrong.) So, I drove the truck around the block and let it idle for a few minutes to charge the battery and did it again. The second test was done 4-3-2-1 and the first was 1-2-3-4. I did this as a control to counter balance test #1. The results of test 2 were : #4-185psi; #3-183psi; #2-187psi;#1-186psi. So, after 278k miles it has more compression than a new engine!!! I now wonder if the high readings are due to a need for a valve adjustment, and if maybe this is causing the rough idle and mild miss at higher RPM's. I have already replaced the whole ignition system down to the distributor with no improvement, so I figure it has to be internal. I also checked the injectors and they all sound like they are firing right, but they are original to the car. The cat and exhaust system are also original, but the car revs fine so it does not appear to have an exhaust restriction. Comments? Suggestions?

Also, the test was done per Nissan's test procedure: engine warm, all 4 plugs out, fuel pump and ignition disconnected with no fuel pressure, throttle wide open while cranking.

 

[megaladon6]

how accurate is your gauge?
carbon deposits can actually increase compression.
unless you have major wear or damage you really won't lose compression
hows the valve lash?

switch the injector from the problem cylinder to a good cyl and see if the code changes. double check the PCV valve. i know it sound weird but it happens. also check the EGR system. if your engine has the annoying method of running individual egr feeds to each runner they are rather small and will clog causing a misfire.

 

[85 Cutlass Brougham]

I have pulled the EGR system apart, cleaned out all of the passages (none were clogged) and reassembled them. I even tested the valve for proper operation. I also changed out the cracked exhaust manifold (common on the KA24DE, I used one from Dorman instead of Nissan as it is built better), the O2 sensor, crank angle sensor, cam angle sensor and coil (with new distributor), cap, rotor, plug wires, plugs, PCV valve, fuel filter, breather filter, air filter, tested the TPS and aligned it, etc. It uses a little oil ( 1 qt every 3,000 miles) but I have traced that to a leaky oil pump gasket that will be changed soon. I have already changed the valve cover gasket, front and rear crank seals and the oil pressure switch because of leaks. I plan on doing a valve lash check shortly and then maybe seeing if I can get the shims to fix it right, and/or the bigger cams from a S13 240sx. The engine internals have never been touched including the timing chains and their tensioners. I may also try getting a few junkyard injectors from a KA24DE powered Altima or 240SX and exchanging them to see if anything changes. It's more an aggravation now than anything, but if we ever get a smog inspection here, it will not pass because of this issue.

The gauge is new but low cost as I got it from Harbor Freight. The lack of oil consumption or blue smoke tells me that it likely is right. I have tested the engine previously with 2 other gauges and gotten similar good results.

 

[87MonteSS]

When you say "new" do you mean brand new, or new to the vehicle?

If you do mean brand new then I would say carbon deposits are causing the problem.

 

[85 Cutlass Brougham]

All of the parts I listed as new I meant in a literal sense. None of these trucks have ever made it into the U pull it yards I frequent, so I have bought very few used parts for it in the 9 years I have had it. Like all Nissan trucks, Frontiers tend to be very long-lived going several hundred thousand miles before getting junked. It seems to take about 12-15 years around here before they start to show up in any quantity. This may be different where road salt is used, but around here you see plenty of GM cars and trucks in the U pull its within 5-7 years while Nissans and Hondas take 10-15.

 

[85 Cutlass Brougham]

Just thought I would post an update: I decided to play with the old POS today and see what could be done about the idle issue. I adjusted the throttle body's cold idle circuit and reconnected the IAC motor I had disconnected due to another issue a few weeks ago. Got it to idle at the right speed, but still had the misfire. I was thinking about what megalodon6 had posted about the small jets in the EGR system and realized I had not cleaned out the intake manifold, just the feed tube and valve. So, I looked at the intake manifold and noticed it has 4 pipe plugs in the EGR part of the casting. I pull out the one in cylinder #6 and probe at it and, no hole. So I poked a little deeper and pushed my dental pick through the hole! Yeah, it was plugged up. I also did this to cylinder #1 and it was plugged up too. I did not do #2 and #4 because I could not get to them without removing the EGR valve and throttle body and I had to work tonight. It idled much better when I initially started it up and seemed to run smoother. Also, no miss though the tailpipe! Unfortunately, after a brief test drive the problem returned. I probably dislodged some particles and they got in another port. So, now I know what I have to do and it is not expensive either. Maybe $20-50 worth of new Gaskets depending on what has to come apart (dealer only) and I should have it running like new! Now if only I could get the engine to be a little quieter no one would be able to tell it has almost 300k miles on it.

 

[megaladon6]

it's a common problem amongst nissans. my brother's a class-a tech and it drove him nuts the first time he tried to figure it out. i would pick it and use a spray bottle of carb/intake cleaner. you will also want to clean the main EGR tubes and maybe the the intake itself. if you have the old plugs, put them back in for this and when everything is clean, but back the new ones. otherwise you run a good risk of fouling the plugs.

 

[85 Cutlass Brougham]

Well, now it runs worse than before I cleaned out the holes! It idles rough as before but has a problem at low throttle positions just off idle where it will skip there too, so I have to run it in a lower gear. I am going to try pulling the throttle body and cleaning cyl #2 and see if I can get an allen key into cyl #4's plug and clean it out too (I have the TB gasket in stock from a previous repair and never used it). I then plan to clean #1 and #3 again then put the plugs back in #2 and #3 then blow out the passage with compressed air by putting the air hose on #1 and leaving #4 open. It threw P0303 and P0301 tonight at 2 separate times which corresponds to the ones I cleaned. So, maybe it fouled those plugs? Unfortunately, I am so poor right now that the price of new plugs will break me financially-and it's under $10. AAAGH! Why do I need to fix stuff when I have absolutely no money to spare?

I also want to find a really cheap electric oil pressure gauge so I can have some idea of what condition the crank bearings are in. I really do not want to spend more money on maintenance of this high mileage engine if it is about to go south. If it is good, I may do the valve lash and a new timing set-provided I can fix the idle issue first. So many inexpensive things to try, and barely enough money to eat on.

 

[megaladon6]

the ones that are cleaned are now getting all the egr flow also. you should be able to clean the plugs with carb cleaner and light sandpaper.

 

[pontiacgp]

the higher compression reading than what they should be could have been simply caused by the ambient air pressure at your location...