# '90 HB won't idle correctly after head gasket replacement



## SBJ (Oct 30, 2016)

Hey everyone,

I have a '90 HB Truck (4cyl, 5spd 2WD) that I recently replaced the head gasket on. While the head was off I removed both the intake and exhaust manifolds and cleaned the intake. I originally was trying to diagnose a skip in the idle once warmed up (the EGR valve was stuck open and totally caked with carbon deposits) but noticed the head gasket leaking oil from the back of the head on the driver's side. Long story short, I replaced the head gasket and had the head cleaned at a machine shop and new valve stem seals installed. After putting everything back together I am able to start the truck but engine idles extremely low and will stall if the throttle is not applied. I have pulled the valve cover and distributor cap to verify that the engine is in time. Once gas is applied the engine revs up abruptly, much more quickly than it did before. Other than the slight skip in the idle the truck was running fine before. Here is a list of everything I have done so far that might pertain to the fuel/air system:

Spark plugs, dist. cap & rotor (2 years ago)
New timing chain and guides (2 years ago)
Replaced fuel filter and cleaned injectors
Replaced EGR valve
Replaced Oxygen sensor
Cleaned throttle body with manifold off engine, replaced all gaskets
Cleaned Idle Control Valve
Cut and installed all new vacuum lines (several were dry-rotted and full of cracks)
Replaced coolant temp sensor
Cleaned MAF sensor
Re-soldered fuel injector power connections inside wiring harness. I did this repair shortly before taking the head off and the truck seemed to run fine afterwards.


I have confirmed that all 4 injectors are getting 12 volts. I have been trying to adjust the idle speed to no avail. With the throttle position sensor unplugged the engine revs upon startup and starts to fluctuate about 10 seconds later. With the Idle Control Valve unplugged the engine will barely stay running unless the throttle is applied. Can any hardbody/240xs enthusiasts shed light on what might be causing the problem? I'm thoroughly stumped, as the truck ran fine before I took the head off. Could it have something to do with the Swirl Control system at the front of the intake runners? The engine is a KA24E SOHC.

Thanks Everybody!


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## jp2code (Jun 2, 2011)

There could be a vacuum line plugged into a wrong hole or left off.

Also, check the timing.

If those are all good, I'd say pull the codes.

[VIDEO] Checking Error Codes - Infamous Nissan - Hardbody / Frontier Forums


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## SBJ (Oct 30, 2016)

UPDATE: Truck is still not running right, but I think I might be onto something here. I pulled the valve cover and distributor cap; both the timing gears and the distributor are in time. I think the problem might have to do with the vacuum line to the fuel pressure regulator. Just after the engine stalls I can see white vapor in the throttle body (classic flooding) and I think the vacuum line to the FPR is routed incorrectly or the one-way near the vacuum tank is installed wrong. This is causing the engine to barely stay running at idle because not enough fuel is being returned the tank. It also explains why the throttle is so touchy -- there's so much gas already in the combustion chamber that even the slightest bit of air will cause it to rev abruptly. I dug up the vacuum line routing diagram for this engine and highlighted the various sections. I can upload a PDF of this diagram if anybody is interested or thinks they could benefit from it. I will check the vacuum line routing next chance I get and see what I come up with.

Another theory I have is that the swirl control valve is staying open at idle, which could most certainly be caused by the one-way valve being installed in the wrong order. This could also be the reason for the poor idle and abrupt throttle response (barely have to touch the gas and the engine revs like crazy). The EGR valve is brand new and all of the sensors were working correctly (got a code 55 last time I checked) before I took it apart. Any other ideas? Thanks!


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## SBJ (Oct 30, 2016)

*Fuel/Air Mixture Screw*

ANOTHER UPDATE: Two of the vacuum lines were in fact routed incorrectly; the lines for the air injection valve (AIV) solenoid and fuel pressure regulator were reversed. According to the diagram I have the fuel pressure regulator vacuum line is supposed to go directly to the throttle body while the AIV vacuum line goes to one of those plastic T connectors. I switched the lines and there is a modest improvement: the truck stays running, albeit poorly. I stupidly tried turning the fuel/air mixture screw (the one you're not ever supposed to touch) and so now I'm thinking I'll need to take it to a shop to have them set the mixture correctly. I can't seem to find any information on adjusting it and I doubt I would have the equipment for it anyway.

With the engine warmed up (I was able to get it warmed up all the way, no overheating) I had a look at the ECU. Mine is a base model with no tach or check engine light, so I have to go by the two LEDs on the ECU itself. In mode 3 with the engine running I get a code 55 - no malfunction. Here's where it gets really interesting: I am unable to get a reading from the ECU in either diagnostic modes 2 (fuel/air mixture) or 5 (real-time diagnostics) as both LEDs go out completely. I am almost certain it's running extremely rich, as I looked under the truck after about 15 minutes of running and the catalytic converter was glowing red. It has never done this before and I'm sure it has everything to do with the fuel/air screw on the throttle body that I wasn't supposed to turn.

Am I better off just getting a throttle body from a junkyard truck and hoping it's still set to factory specs?

The truck stumbles at idle (RPMs are really low) and revs sporadically before dropping down and stumbling again. There is also a delay between when I release the throttle and when the engine speed drops. I am beginning to suspect I might have damaged the Idle Control Valve when I was cleaning it, but I'd like to rule out other possibilities first. I have confirmed that the vacuum lines for the other two switches are all routed correctly and that the solenoids themselves are working (EGR control and Swirl Control). The throttle position sensor also seems to be fine, as the voltage changes linearly with the butterfly valve position.

Any ideas? Thanks!


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## CMax03 (Jan 17, 2009)

Check your timing, TPS setting, recheck your vacuum lines, check all your sensors at and returning wiring to ecu...if all that fails check your compression and valve clearances...


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