1989 d21 vg30i surging idle, imput please.

[72240z]

Hi everyone, long time reader 1st time poster. Actually signed up just because of this problem.

Its a pretty frustrating problem. You start the truck with no problem at all, the idle shoots up to about 2k as normal(cold) but as soon as it starts to warm up and drop the idle will suddenly shoot down to 600ish then rise to 1k then fall over and over. A look into the tb shows what seems to be both injectors firing full then cutting then full then cutting over and over.

My work horse, 1989 d21 with a vg30i. Truck has 220k miles on it, about 10k ago I swapped the motor with a 88z31 motor(70k) but retained the stock d21 electro injection. Eliminated the egr and pump, added headers. Ran like a dream for a few thousand miles then all of a sudden the surging idle problem developed. I ran the codes, det sensor, maf sensor, cas codes came up. To my knowledge (and all datas too) the truck never came with a knock sensor but I added one and the code went away, maybe I'm wrong Idk. I replaced the maf with a used one due to the cost and that code disappeared as well. Replaced the entire dizzy with a z31 dizzy using a d21 cap and that code disappeared. The surging idle did not though. I checked the tps with a spare I had, nothing. If you leave it disconected the idle shoots to about 2k and no surge. Checked timing with a light, on the money. I changed the IAC valves with one I had from the z31 plenum (All data says they run the same current) No change. The air filter is clean. There is a bit of a gas smell in the exhaust although no smoke so its hard to tell if its running rich and I should replace the fpr or not because the egr and pump are gone. I have run the codes a few times as well as disconnected the bat to reset the ecu. I get no codes(before the reset of course:-)). I also tapped both injectors and checked the current, they seem fine. I also discovered while checking around with the ohmeter that the mixture heater is not working. There has always been a blown fuseable link (since I bought the truck)that I paid no mind since everything seems to work, all date shows that is the heater.

So thats where I am at. I am thinking fpr but at this point with all I have checked and been through I am asking for advice 1st. Just about had it to be honest, feels like maybe something stupid I'm missing.

I know there are many knowledgeable people here and someone is bound to help so thank you VERY MUCH in advance.

P.S. Sorry to ramble I just wanted to include everything I could think of to aid in diagnosis, thnx again.

[nismopu]

Theres a plug that sits on the passenger side of your intake whos harness leads all the way behind the cam gear. Down there is whats known as the CHTS (cylinder head temp sensor) it looks just like a coolant temp sensor. Anyway, unplug the harness and check the resistance on it and make sure its within spec. It will cause all three of those codes to appear and all kinds of strange running problems. Also, if its not even hooked up your vehicle wont start. Its a real pain to change so when you do it cut a small notch in the timing belt cover so you can get a socket on there easier in the future. peace.

[72240z]

Hi, thank your for you input, completely forgot to check the chts. I just checked resistance now as per manual specs i.e. 68degrees = 2.3-2.7, 122 = .77-.087, 176 = .30-33 and it checks out almost on the money. I also checked every vac line and sprayed around everywhere with carb cleaner to check for a leak but found nothing at all.

Any other suggestions? Thank you again.

[nismopu]

Ive heard maxima guys have this problem with the heaters all the time but usually theyre cars dont stay running and have to feather the gas. FPR would sound about right for the gas smell, but doesn't it drive ok? Maybe pull the vacuum line off to the FPR and see if the idle changes with the rise in pressure. Also, check the wire connections at the battery to the ecu, we had something similar with a v6 truck in the past. peace.

[72240z]

I checked the fpr, seems ok, vac line changes doesn't effect the surging and I don't smell gas coming out of it. The bat terminals are good, I replaced them when I got the truck because it had starting issues. It drives perfect besides the embarrassment of this idle and some squeaks lol.

One think I checked after the fact is the voltage coming from the chts. Alldata says voltage should be .1-.5 cold-hot, I assume that means gradual as the truck warms up the voltage rises and the rpm drops. When I check the voltage though its fluctuating 1-2-3-4-0 1-2-3-4-0 in sync with the rpm surging. What I don't understand is why if thats the case the surging doesn't happen when the truck is 100% cold, not until it warms up a tiny bit. Does something over ride it initially?

In any event I went and pulled on off a 90's quest and plugged it in just to see if there would be a change. Rpms with engine hot stay up at like 2k for a min then the surging starts but from 2-2.2k instead of the normal 600-1200ish. Maybe it runs in a dif range or something Idk.

Opinions? Thnx again

[72240z]

Its sad only 1 person tried to give an opinion in over a week and 105 views. Says a lot for this forum........

[nismopu]

This forum has alot of other helpful sections in it. Theres a specific Nissan tech one where you can post this question with an actual online mechanic. This honestly is the best place to find the info your looking for. I find it unusual none of those things helped narrow it down better. I wonder fi you have checked voltage going to any other sensors when this happens? It sounds oddly enough ;ike theres a power issue. I cant explain how without looking at the vehicle myself. I do suggest posting this thread in the nissan tech section as well though. Sometimes takes those guys a few days to respond but theyre usually pretty helpful. peace.