1991 B2600i

I need some help. I am working on B2600i. Just put a new head on it, runs great. These are the things I am struggling with:
1. Speedometer is jumping and not accurate. It looks to be as much as 30 miles more and sometimes 10 miles more than the speed I am running. Cable was removed and greased. Problem remains consistent.
2. When I put the truck in gear I hear engine noise towards the front of the engine around where the head meets the block. It is noticeable especially if you are in front of the truck and someone puts the truck in gear while you are standing there. Almost like putting a load on the engine causes noise.
3. Mileage seems to be around 15 mpg. I see where some folks post of mileage up to 26 mpg.
4. Wandering idle. Sometimes idle will remain around 750 in and out of gear, and then sometimes it will jump to 1500. Especially on cold start up, the rpms go past 1500.

Any help is appreciated.
PS - Thanks to Dan. I bought some parts from him. Great deal, exactly as described.

1. Was the speedometer cable all gooked up with oil...turned to grease? Grease on the back of the speedo cluster that got pushed out the cable? If so, then you need to clean the grease from the speedo head. Was this truck sitting for a while before you got it? Here is a picture of a speedo assembly that had gear oil pushed up the speed cable and got inside the speedometer.



Cleaning it from the fitting where the cable attaches won't solve the problem because it's the inside that is gummed up.

2. Automatic transmission? Maybe a thrust bearing in the engine is bad and your getting some movement. Maybe have somebody watch the harmonic balancer when you shift in and out gear and see if you can see any forward or aft movement of the balancer relative to the engine.

3. 15 mpg stinks if that is your mileage on the highway with a 2wd truck. Not great if it's a 4x4. How many miles? O2 sensor ever been replaced? Brakes dragging? Check ECM for signs of leakage or previous repair? Check timing? Check FPR vacuum plumbing and operation (vacuum lines are commonly cracked, leaking, missing, disconnected, or hooked up wrong - If your FPR is not getting vacuum, you will be running higher than normal fuel pressure.)

4. Check Coolant and temp sensors to make sure they're in spec. Check IAC valve for proper spec. Also try unplugging it. Check if throttle is fully closed or cracked open - You should have a little slack in the throttle cable at idle. Try giving it a little twist by hand to see if you can force it closed. I have seen a throttle that was worn out and sometimes it would close all the way and other times it wouldn't. It doesn't take much air leaking past the throttle to get you from 750 up to 1500 rpms. If you can twist it closed and make the RPM drop back to normal, then you might need to pull the throttle and try cleaning it and checking to see if you can see any daylight around the butterfly when closed. Check PCV valve. Have this idle issue before the head replacement?

Any other background on the truck would be helpful too. Idling like this before the head R&R? Any bottom end work? Other issues or repairs?

I really appreciate the information. Truck has not been sitting.
1. When I took the cable off of the back of the speedometer it was full of grease. I will disassemble and clean it. Nice pic.
2. I will check the harmonic balancer while someone puts it and out gear.
3. Ok. I will check vacuum on FPR and ECM. O2 sensor, cap, rotor wires, plugs, temp sensor are new. Mileage remained the same 15 mpg. '
4. I previously had cleaned the throttle/butterfly. it does not appear to be loose o not closing. I will check the IAC. PCV is new and the vacuum line to it is new.

280,000 - plus miles on it. Just did the head. Each cylinder has at least 150 with two having 154. it runs great as take off and power, but just idle wanders and the noise coming from engine when in gear. Idling has always wander even before head repair and truthfully mileage was just as bad. I thought mileage would improve with all the changes. I will start looking at all vacuum lines. Thanks again. I will update by Saturday.

Wow. Sounds like you have gotten some miles out of her. Congratulations!

Just to confirm, this is 2wd and automatic?

When you take the cluster apart, the black trim comes off WITH the clear plastic lens. Don't try to separate the clear lens by itself, else you'll crack it. You'll probably see some grease inside that copper area too. I used some electrical contact cleaner and that small straw to blow and wash it out.

I'm not sure if high fuel pressure would sink you mileage that low or not. But as I mentioned, the vacuum lines frequently have issues mentioned above... especially on 4x4s which have a few extra lines.

I would also check out the MAF sensor and make sure that 1) the connecters are clean and in good shape, and 2) that is is within spec with engine off and with engine running. Here are the tests/specs for most of the efi components:

You might also look at this thread:



How about the resonator o-ring and rear vent tube?

It's sounding like it's going to be an IAC or ECM issue, but check all of the components you can.

1991 B2600i 2wd automatic Cab Plus

1. Speedometer. Took it apart and found that it was full of grease in the back. Using a GPS I am now within 3 to 5 miles accuracy.
2. Engine noise - I had someone put the truck in and out of gear repeatly and the harmonic balancer is rock solid. Still is noisy in gear. Could the idler pulleys cause that kind of noise? They have some play and are as tight as I can get them.

3. Lousy fuel mileage - I have not had a chance to check it. I plan to buy or borrow a timing light and get a base line by setting it on time. I also will ground the green connector to set the rpms.

4. I removed the IAC, cleaned it and installed a new gasket. I also replaced the vacuum hose on the FPR.

I will not be able to do much to it for the next couple weeks, but i will keep you guys posted.

1991 B2600i 2wd automatic Cab Plus

1. Speedometer. It started acting up again. I took it apart cleaned it again. Now i am within 4 miles.
2. Engine noise - Replaced the two idler pulleys. The noise went away for the most part. It does have a little but not like it use to be. I think I just had bad pulleys. Set timing at 6 degrees and then to 4 degrees. Not sure which is best.

3. Lousy fuel mileage - Set timing using the procedure of grounding the one green wire by wiper motors. I am up to 17 miles.

Only really question is, Should I be able to squeeze more out of it?

It is FI? If so,clean ur MAF with some electric cleaner. Remove the rubber boot to ur intake and clean ur throttle body with Carb cleaner. 19-20 mpg on a 2600i is about all u can expect.bj

I want to thank you guys for the input. I have the truck running like a top. I am getting about 18 mpg. Idle is smooth. Let me list everything I did to it:
Replaced cylinder head.
Replaced water pump.
Replaced fuel pump.
Cleaned MAF
Cleaned idle air control
Replaced air intake hose from MAF to throttle body
Replaced wires, plugs, cap and rotor
Set timing per correct procedure also set idle.

What a difference!

Glad to hear that. Sounds like you're good for another 280,000 miles! Thanks for the update.

18 isn't too bad for an automatic. I get between 18-20 in city driving with my 2wd Cab Plus with a 5 speed transmission; 20-22 on the highway, maybe a couple more if not on the interstate where the flow is 75mph.

How were all those coolant lines around the back of the engine? You don't want to take any chances with that new head.

I replaced all the coolant lines while I had the head off. I could not see putting it together without doing it. The tech that designed the coolant system must have had stock in coolant hoses. I posted two other threads.
Ignition lock set
no heat in cab

Any ideas will help